ROD vs Groundspeed
Agreed.
However, if you teach your student to calculate ROD based on ground speed for an ILS, it should be equally effective.....
rule of thumb = for a 3 degree Glidepath use 5 x Ground Speed and so on.
EG: Ground speed 80 kts, ROD required = 400 fpm.
On a non-precision approach you can calculate the ROD required fairly easily. Normally on a light general aviation aircraft it is desirable to reach MDA by approx 1nm before MAPt. Therefore, if the MAP is defined by timing, calculate your rate of descent based on feet per minute. If it is based on DME, calculate it based on feet per nautical mile, (then from there you can always convert it into feet per minute). Airlines will generally fly a 3 degree path from FAF to Mapt, add 50ft to MDA, and treat it as a DA.
As a general rune, I usually teach my students to have an altitude check half way between the FAF and MAPt. If they are slightly high, increase ROD by 100 fpm, if slightly low, reduce ROD by 100 fpm (obviously respecting any 'not below' altitude fixes).
Should get your student thinking and planning more about the approach on the ground, which will lead to better understanding of these concepts and ultimately better performance in the air.
Hope this helps.