Yeah it works in 2.83, I just want to make it a little more convenient.
At the moment you have to go into tool settings and click it on and off. Once I figure out how (or someone shows me how) to add that part into the keymap, we’d be able to map it to whatever we want.
To get it working just go through all the scripts at the end of that link
https://developer.blender.org/D6322
I found it easier by clicking the ‘view options’ and choosing ‘view standalone’
It will show you each script and where it’s located. The original is on the left, and the new one on the right. Green stuff on the right is additions, red stuff on the left is subtractions. Just have that one change to the iterators script that I pointed out.
Everything else is fine, but most of it is on different lines.
Best I can think of right now is to make a custom operator in python that basically just “clicks” that checkbox for you, but I was thinking more along the lines of having it in the keymap where you can map it to a modifier the same way you do the different modes like extend, subtract, intersect, etc
wm_operator_props.c
looks promising
Yeah, sure enough, adding that will put a “select through” checkbox under “wait for input” but at the moment it doesn’t actually do anything. Now to figure out where to put stuff so it connects up with it like it should.
Ok so the guy who actually did all the work on this says
“The setting itself is setup as a toolsetting and not as an operator property, mostly because it was easier for me to access it in the draw code (needed do decide if we draw the facedots in xray mode).”
I’m noticing stuff saying
if (blablabla)… ts->mesh_select_through
So yeah this will probably require some actual work. Going to go through this and hope for the best. Not expecting much though honestly, don’t know much about this stuff.
wm_operator_props.c
this is where you can add a select through property in
void WM_operator_properties_border
under wait for input
prop = RNA_def_boolean(ot->srna, “mesh_select_through”, true, “Select Through”, “”);
RNA_def_property_flag(prop, PROP_HIDDEN | PROP_SKIP_SAVE);
it will add a checkbox for select through in the keymap, where I want it to be, but it needs more to do stuff.
I think it will work like I want if I make it check for this property in the relevant parts of view3d_select.c where it currently checks for ts->mesh_select_through (the tool setting of select through)
I don’t know how to do that. How to make something check for a property in another script? I wish there was something like a cheatsheet for doing this. You know, what magic words to say and which Include stuff to throw at it. Going to fumble around some and figure it out one way or another. How hard could it be? 
So it’s back to the waiting game…