Once you have established the IR-focus mark (by whatever formula or by test exposures), you only have to shim the groundglas (or the viewing lens) to that new focal plane. From then on you can use your TLR as before:
Originally Posted by EASmithV
Establish that new focusing mark.
Focus a target of any kind visually without filter.
De-focus the taking lens so that its focusing mark alines with that IR-mark.
Shim the groundglass or de-adjust the viewing lens until your groundglass image is sharp again for that test target.
Now you got a IR-only TLR (of course do not forget to mount that IR taking-filter).
In case you did the method of de-adjusting the taking lens, you might be able to adjust that lens between two marks. One for visible, one for IR-light, then you even got a dual purpose TLR.