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.