SoftICE?

Does that work with protected mode programs? (dos4gw bound)

I used SWAT for the debugging of SoftMPU which uses protected mode.
http://www.sudleyplace.com/swat/
I think it has the features you look for.

This seems just perfect, thank you!
However I seem to be doing something wrong since I keep running into invalid opcodes and meaningless code while single-stepping. I need to find the point where DOS4GW finishes doing whatever it does (and before the game hijacks the keyboard interrupt) and resume debugging from there. But how do I get there?
I also tried setting breakpoints at the I/O ports I'm interested in, using the debug registers. But these get trapped by DOS4GW instead of SWAT 🙁

I don't know. It worked fine here. For SoftMPU I used it in combination with EMM386 4.46+ and QEMM 7.03+ (up to 9.0) on a 486DX2-66.

After wasting several evenings on this I finally figured it out. 386SWAT kept crashing when stepping over the function call that initializes protected mode, and setting breakpoints on the other side just started the game and didn't do anything. I/O breakpoints were all intercepted by DOS4GW which then crashed instantly.
But then I realized OpenWatcom has a debugger too... Of course! It should be designed to work with DOS4GW, after all. And yes, running the unbound LE exe in Watcom Debugger dropped me right at the game's entry point.
The dual-screen function was a bit pointless in the end since the game does its sound initialization stuff right before switching to graphics mode. It did allow me to run WD on one screen and 386SWAT on the other, though, which works great (I/O breakpoints set in SWAT are then handled by WD)