No problem.
I tested with the default DA20 and 40 and analog C172 as these have the GNS530/430 and VOR with glideslope indication (mind though, they do not have an HSI).
When performing an RNAV approach, the localizer indicator kinda moves as expected (though is much less sensitive as when on ILS) and the glideslope needle does not move at all (tested with the modded GNS530/430).
Interesting thing is that when on autopilot (C172, the DA20/40 do not have one), the aircraft follows down the glideslope even though it is not depicted on the gauge. So the RNAV glideslope is actually there and the autopilot is able to catch and follow it, it is not depicted on the VOR/HSI.
Also, as mentioned above, the localizer needle deflection is very weak compared to ILS (or to other sims).
So this issue is probably not linked to the JF Arrow III only (I tested also the Lionheart's Trinidad and the result is the same). I will file a support ticket anyway.