SOLVED.
It turns out that the two input libraries present very different device properties to X, so I don't think it will be possible to replicate the TrackPoint behavior exactly.
This is in TCL Pure64 11-beta using the default xf86-input-libinput:
bruno@box:~$ xinput --list
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ TPPS/2 IBM TrackPoint id=10 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Power Button id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=9 [slave keyboard (3)]
# note that id of TrackPoint is 10
bruno@box:~$ xinput list-props 10
Device 'TPPS/2 IBM TrackPoint':
Device Enabled (181): 1
Coordinate Transformation Matrix (183): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000
libinput Natural Scrolling Enabled (316): 0
libinput Natural Scrolling Enabled Default (317): 0
libinput Scroll Methods Available (318): 0, 0, 1
libinput Scroll Method Enabled (319): 0, 0, 1
libinput Scroll Method Enabled Default (320): 0, 0, 1
libinput Button Scrolling Button (321): 2
libinput Button Scrolling Button Default (322): 2
libinput Middle Emulation Enabled (323): 0
libinput Middle Emulation Enabled Default (324): 0
libinput Accel Speed (325): 0.000000
libinput Accel Speed Default (326): 0.000000
libinput Accel Profiles Available (327): 1, 1
libinput Accel Profile Enabled (328): 1, 0
libinput Accel Profile Enabled Default (329): 1, 0
libinput Left Handed Enabled (330): 0
libinput Left Handed Enabled Default (331): 0
libinput Send Events Modes Available (301): 1, 0
libinput Send Events Mode Enabled (302): 0, 0
libinput Send Events Mode Enabled Default (303): 0, 0
Device Node (304): "/dev/input/event5"
Device Product ID (305): 2, 10
libinput Drag Lock Buttons (332): <no items>
libinput Horizontal Scroll Enabled (333): 1
# the above properties look very different than what I see in TCL 10.1 with the default xf86-input-evdev
# note that Accel Speed is property #325
bruno@box:~$ xinput set-prop 10 325 -0.4
Now TrackPoint is better behaved. Not identical to xf86-input-evdev defaults, but tolerable. I'll get used to it.