I can't help with the DOSBox side of it, but there is a great mod for the original System Shock that gives you (pretty much) standard FPS controls. The person who made it went on to help develop SS Enhanced.
- Download Malba Tahan's mouselook/key mapping/high resolution mod here: https://www.systemshock.org/index.php?topic=1719.0
- You'll need to grab both zips (XCDSHOCK + XCDSHOCK206 update)
- Drop XCYB.CFG, XCDSHOCK.EXE and XCDSHOCK.BAT into C:\SSHOCK or whatever your install folder is
- You also need to drop a copy of dos4gw.exe in. The mod changelog says it's no longer needed, but I needed it.
- You can edit bindings in XCYB.CFG, but the defaults are pretty good. (You may want to change mlook_vsens 40 to -40 to invert the mouse, and also change the mouse buttons so left click attacks instead of right).
- Run the game with XCDSHOCK
And with that you'll have a very FPS-like experience in the original System Shock on original hardware. Use E to toggle between mouselook and interact.