Per
A few additional comments in response to some of your outstanding questions.
"I
do not see ICC profiles for QTR in colorspaces when exporting, so it
seems like these icc profiles are not installed per default in windows
when installing QTR." They're not installed to the Windows directory for ICCs when QTR is installed. Walker mentioned QTR_RGB_Matte and QTR_RGB_Gloss, and you'll find them in C:\Program Files (x86)\QuadToneRIP\icc and you'll need to copy them to C:\Windows\System32\spool\drivers\color . Note that the Grey versions of these won't show up in LR, only the RGB versions. Both should appear in PS.
Since you have the ability to linearise a QTR curve, you also have the ability to create matching custom soft-proofing profiles. In C:\Program Files (x86)\QuadToneRIP\Eye-One you'll find QTR-Create-ICC and QTR-Create-ICC-RGB which you can use to create ICCs from your measurement data. The idea is to reprint your test chart with the linearised curve, measure and create the ICC.
Soft-proofing is usually regarded as a key part of colour printing, and IMHO it is also for monochrome. There is however one potential difference, in that not everyone uses the ICC to print with, some only use it to soft-proof in the so-called linear workflow. More details in a tongue-in-cheek blog entry I wrote on this topic. It's aimed primarily at Piezography users, but the principles apply to other inksets. Note also that the "preserve numbers" option used in the linear workflow means soft-proofing in PS - you can't do this in LR. You mentioned in an earlier post that you are aware of this option, also mentioned by Walker.
"I have read in the 21 step wedge that is included in QTR (which is
untagged), and inverted and saved it for my experiments. The saved
version is still untagged. Sample values of the steps have the right
percentage black using the eye dropper. Should I have done something different? " I'm not qualified to comment on digital negatives, but for paper printing the answer to your question is no. You don't need to do anything to the 21 step wedge, you can leave it as untagged, on the assumption that you're printing it through QTRGui. QTRGui is not a colour managed application and will ignore any embedded ICC profiles. You could assign it to GG22 and that would also work. But what you don't want to is convert it to GG22 if your default working space is 20% dot gain. It's not clear why you'd want 20% as your default working space. As Walker mentioned, GG22 is a more convenient and safer option if you're primarily printing monochrome through QTR(Gui). If you want to keep your image as RGB then use AdobeRGB, which also has a gamma of 2.2. Again, this is for paper, others will have to correct me if dig neg requires something different.