The manual says that setting XX/ in the flight plan aircraft type sets the NRAC, but trying it both in the TopSky or the default flight plan windows it doesn't take the XX/ bit. I Think this has been broken for a long time, maybe even back in the 2.3 versions

The TopSky FPL Window's NRAC edit box doesn't seem to be connected to anything, that's a bug, but the number can be set using the default flight plan dialog - as long as it's not greater than 9. It appears that numbers 10 and greater are not parsed correctly by EuroScope and are taken as the aircraft type instead.

Even through the default window, setting for example 2/B738/L does nothing. There's a '+' shown for a fraction of a second on the label, so maybe something is resetting it

Something probably is, but it's not doing it for me. EuroScope seems to get easily confused - "2/B738" leads to "2" being seen as the aircraft type (any two-digit number fails as well), but "2/B738/L" works just fine for me on EuroScope 3.2.9.

If something on the network is removing the number prefix, then that's it. The code will remain in place should it stop doing that some day, but until then this feature - fortunately not a key one - is not going to be available.