It appears to be common practice to bury the graft union when planting out Feijoa. The reasoning is that when suckers come up, they are of the scion variety, not the rootstock.
Ram, I am not an expert in feijoas, however I think if the rootstock is too deep and there is no light, it will not emit suckers (those are usually coming from ground level or about 1-2inches below the ground).
It can be applied for sure. In my country there is even an expression that loosely translates to “truerooting”. It makes sense in many situations, eg. you have a cold sensitive plant like fig or Asian persimmon that is grafted but you know it can be killed to the ground and then only the inferior variety (or in case of persimmon lotus or virginiana) shoots from below will grow so you can scratch the bark on the grafted part (this is important, like in the case of rooting) and bury it deeper. Or in the case you want the plant roots to have characteristics of the variety and not rootstock eg. you are having difficulties rooting bush cherries so you graft it onto cherry rootstocs and bury deeper with the bark scratched the way I described above. Or with other fruit like apples. You want to have vigorous tree but all you can buy is a dwarf on M9. Or as you mentioned with loquats, yours is on quince but you want it to be a bigger tree, like a seedling. Way to go in that cases. That’s also the reason why nurseries advise you (when you plant a new tree) not to bury below the graft union cause if you do it can emit roots off the grafted variety and it will lose the beneficial characteristics of the rootstock. Not sure if that answered your question, maybe someone else will chime in.
I heard back from a Feijoa grower not in this group. He said that burying graft union completely (or almost completely) suppresses suckering of rootstock. The top part may root a bit, but not very significantly.
The same technique will work with other trees as suckers created will be the desirable top part.
This might be an excellent way to go with figs that automatically sucker a lot – graft low and bury deep.
I was instructed by Patrick Schafer to bury graft union when planting grafted feijoas that I bought from him. This worked well for me — I let my feijoas to grow as bushes and don’t need to worry about suckers from the rootstock.