Some audio interfaces will act as a standalone mixer. Some of the MOTU and RME units come to mind. I believe the MOTU Ultralight and the RME Fireface UFX will act as standalone mixers. With the Fireface you have to sort of set up a few preset operation configurations (routing and such) using your computer and it's software interface, then you can just select a mode of operation with the standalone. The Mbox2 will not operate standalone, none of the Digi/Avid interfaces will.
For the cost of the Fireface UFX interface, you can just buy a Sound Devices 702 and have a bit of change left over for extra CF cards and a battery. Besides that, I wouldn't want to take a Fireface UFX out of the studio if I had one. I think the MOTU might be an affordable option though. Another would be the PreSonus FireStudio Mobile.
However, this whole interface option could be a potential hassle like @g.a.harry said. It depends what else you have in your arsenal and whether you work with a boom op or solo.
Personally, just recently did location recordings with a laptop. The one thing that becomes necessary at that point is a boom op and several wireless setups as well as a shotgun or two that are battery powered.
When I did it I had my Mbox2 (2 inputs only) and a Marantz PMD660a (2 more inputs), a few wireless trans/rec pairs, 2 lavs and a battery powered Audio Technica AT897 shotgun condenser. What I was able to do was buy a small XLR - 1/8trs converter cable at the local audio shop so I could get my boom-op to be mobile. I then used two lavs, one on each of the main speaking characters. I was able to keep everything discreet without having to do any mixing besides watching the levels. I also had the output of the Marantz feeding in to Pro Tools so that way I had a backup recording of the Field recorder which acted as an additional sync reference. I was able to find a place that wasn't going to be filmed for the entire day and I set up about 10-15ft away with line of sight to the production so I could stay out of the way, but stay involved. I have to say it worked out really well. It was probably the most relaxed and smoothest time I've ever had doing location audio. With a decent rig I could very well set up everything wireless and sit back at a small foldout table and chair all day watching levels and cataloging takes. The recordings turned out quite decent as well. The Marantz is a bit noisy, but it's not anything iZotope RX can't fix if need be.
Good luck, let us know what you decide on.
Oh, one added benefit of using Pro Tools and a laptop to do location audio is that I can change my track names before recording each take and Pro Tools will name the audio file after what the track is named and then it adds a number, which if you plan out your naming conventions properly, you can even use that as the take #. So if you have the time or the speed to do this between takes, this saves you a lot of file renaming and sorting work later on.
An example of how I"ll name a few tracks in PT for location recording. I'll drop markers as I'm recording to notify clips/overs, background interferences or anything else that can be fixed. (I do this as well as writing it all down.
I might also have a "Notes" track, where I will create a blank/empty consolidated region and then rename it after anything I want to wildtrack or whetever. ie: Notes_01_WT_Knife_Flicks. All I have to do is make a selection, hit Shift+Opt/Alt+3 to consolidate and then hit Shift+CMD/Ctrl+R to rename and just amend the end of the Notes_##_Info Goes Here. I'll use stuff like WT for wildtrack.
Say the production is Dark Knight:
Translation (Pertinent info in parenthesis): (D)ark (K)night_(Lav)alier(1)(S)cene(8)/Shot(A)(T)ake_and pro tools will add the number.
Here's how I will name the tracks:
*Need to be renamed on the fly.
Once I hit record it names the regions after what the tracks are named and will sequentially add a number at the end starting with 01, I use this as the take number and when we switch to a new take, while camera and lights are setting up I switch the track name to the new (S)Scene# and Shot Letter. Once you start recording, since you are using a new unique track name Pro Tools will start over at 01. I have them do the roomtone at the end of the takes so I don't mess up my numbering system.
When you go on break or you're done for the day you can select everything in your regions bin and hit Shift + CMD/Ctrl + K to export regions as files to back them up. It also makes it dead easy to use a thumb or external bus powered drive to give the production manager or director your production audio dailies, that way someone else has a backup as well and they're already renamed and ready to go right there on location.