I don't think attempting to do an ATX for the patch is a good idea. That approach relies on the exact layout of an executable. As soon as there's an update, it's rendered unusable and has to be updated again, painstakingly figuring out new code locations. It's a viable approach if you have an EXE that's around for a decade or so without any updates, but otherwise less so.
If there are specific functions from ATX that are needed, I could maybe look into adding those, but not all of them. Actions might be doable, if I can get some pointers of what these additional actions are.
I could not get the 3-object-storage to work, after enabling it. How is this done?
Have you enabled EnableDualStow in trespasser.ini? I would suggest you look at doc\trespasser.ini for a description of how it works. It's not a 3 object storage, it's 2 stowed objects and one in hand.
The short version is, that there's an additional modifier key. If you don't press that then object switching works as always. If you do hold the modifier key (which is either the key also used for rotate arm or rotate wrist) while switching object, it will attempt to only stow the handheld object. However if there already is a stowed object in that slot, it will still do a switch.
how is anti-aliasing turned on?
Also done in trespasser.ini, with the MultiSamplingLevel option. Look at the doc\trepasser.ini one for descriptions of all the options.
jumping works as in the patched version
Jumping is indeed one of the things that had the v1.1 changes applied.
tried to switch to software mode and it doesn't seem to be listed
Software mode is only supported in legacy DX mode. That means you have to disable DX9, by disabling UseDX9 in trespasser.ini.
Not recommended, but it's there. You can also use HW rendering but disable texture filtering (TexFilterD3D=0) in trespasser.ini. That will make the game look very similar to software, but with all the hardware benefits.
My tpass.ini file says that it's rendering textures at 256, however it doesn't appear to be
It does work in general. You can clearly see the difference between 128 and 256 when comparing two screenshots of some arbitrary level location. That doesn't rule out some special case bug. It could also be the result of some other setting(s) or something caused by it requiring ATX, since you're missing shadows as well.
Is this a level I can download and see if I get the same result?