No matter what other system I think of using to fuel Harnworld, and even though Harnworld was initially created as a systemless setting, I come up against a brick wall every time. Harnworld's Magic and the structure and philosophy therein.
For example, Open Quest. I very good stripped down version of Runequest. It has Divine Magic, Battle Magic, and Sorcery. Basically. But then how do you translate that, or any other system of magic into the magic concept of Harnworld? For me, the specific problem is the six convocations. Translating Battle, Divine, and Sorcery into: Lyahvi, Peleahn, Jmorvi, Fyvria, Odivshe, and Savorya
How do you resolve this situation, if indeed you've tackled it?
I've wrestled with this issue, and have the following two thoughts to offer:
#1) If you're seeking to use a system that has elemental-based magic (like GURPS, Burning Wheel's Sorcery rules, or even Ars Magica), where spells are grouped together based on the 'element' or class of objects (e.g. plants, animals, etc.) they affect, adapation of those rules to reflect the six Pvaric convocations is pretty easy. For instance, just say that mages are limited to casting spells of certain elements based on the Convocation they have chosen to be. For example, just say Odivshe mages can only use "Water" spells, and maybe any light/dark spells that pertain to darkness, etc. This can be done on an ad hoc, basis, or it can be done with a more formal writeup... but the key thing, IMHO, to avoid the temptation to re-write the system's magic rules in an attempt to turn it into Harnmaster Magic. Not only is that a lot of work, it's not necessary. All you really need is something to mimic the basic feel/function of convocationality.
#2) If the system's magic rules are not elemental-based, adapation can either be easy or impossible, depending on what those rules are. If you're playing a game where magic is largely open-ended and/or effect-based, you can do this simply by requiring that a magical effect be described in a convocationally plausible way. For example, in Burning Wheel's "Art Magic" option for rules, a mage uses magic to achieve one of 10 desired effects, one of which is to create a sorcerous blade. One could just require that in this system, the mage specify, "I'm creating a flaming scythe (if Peleahn); an icy spear (if Odivshe); a heavy cudgel (Fyvria), etc. The mechanical effects would be the same no matter what-- the convocations would just provide 'color/flavor', rather than have mechanical ramifications.
On the other hand, if the system provides highly detailed mechanics for magic that seems to have a completely different basis (e.g. summoning/binding of spirits/gods/demons) than the sort of elemental spell-casting, it may be that it really is incommensurate for Pvaric convocations. In that case, the most effective options are probably either NOT use that system and to instead use one that's based on some sort of elemental principles; or just to say farewell to the convocations and use the system as written, accepting that it will result in a slightly different kind of feel to the arcane elements of the Harnworld setting.