I would also be very interested in using the Y-axis for probing on my belt printer.
I've already got the BLTouch running but now I am having problems using it for probing.
What I have done so far:

Setup for BLTouch on a SKR Pro V1.2 running RRF:

M574 Y1 S2 ;Setup Y as probe ; Y-Probe M558 P9 H6 F120 T8000 X0 Y1 Z0 C"^probe" ; enable Y probe, set dive height, probe speed and travel speed M557 X50:170 Y0:10 S20 ; define mesh grid G31 X34.4 Z9 Y0 ; BLtouch offset M950 S0 C"servo0" ; Setup servo 0 as servo port on SKR

Homing Y is a workaround because probing does not work at the moment

G91 ;relative positioning G1 H2 Y-15 X15 F50000 ;lift y relative to current position G1 H2 X50 Y50 ;Move X M401 ;deploy probe G1 H1 Y-285 F2400 ;move Y down until BLTouch is triggered G90 ;absolute positioning G1 Y4 F50000 ;Backoff 3mm G4 P500 ;wait for 0,5s M401 ;deploy probe G1 H1 Y-285 F200 ;move Y down slower until BLTouch is triggered M402 ;retract probe G1 Y10 F50000 ;lift y

Using G29 and G30 moves the Z axis, which in my case is the belt, so probing can't finish as the y axis rather than the Z axis has to be moved.

I've tried a custom bed.g in combination with calling G32

M561 ; clear any bed transform G90 G1 X170 Y5 G30 P0 G1 X140 G30 P1 G1 X110 G30 P2 G1 X80 G30 P3 G1 X50 G30 P4 G1 X20 G30 P5

But that also doesn't work out unfortunately.

I am thinking of changing the motion system to CoreXZ instead of CoreXY just for the purpose of probing and then switching back to CoreXY but that would probably mess up the heightmap for using it with Core XY kinematics. Is there maybe a more convenient and more simple solution for this problem?

Is it maybe possible to create a heightmap sort of manually?
For instance using

G1 H1 Y-285 F2400 ;move Y down until BLTouch is triggered

saving the trigger height as point 1 and then move the the next point and do the same steps multiple times until you get a decent heightmap of the y axis (5 would probably be sufficient).