There are other ways for experience to be relevant to the justification of a priori beliefs than by enabling the concepts that are part of their contents. It's also standardly allowed that certain experiences would defeat a priori justification -- the experience of lots of misleading but authoritative testimonial evidence, for example, could make it unreasonable to retain some particular a priori belief. If such obtained, then you wouldn't be justified; therefore, your justification depends on such experience not obtaining. Thus does, for example, Marcus Giaquinto distinguish between 'positive' and 'negative' roles for experience:
We could mark the distinction by saying that if a belief is rationally revisable in the light of future experience, its retention is negatively dependent on experience; and if a belief cannot have been justiﬁably acquired unless some experience was used as grounds in the process, its acquisition is positively dependent on experience.
But there is room for a kind of dependence on experience that is neither 'negative dependence' nor 'positive dependence' in Giaquinto's sense. And I think that it, too, is consistent with apriority. We can distinguish between the reasons in favor of some belief, on the one hand, and various conditions that are necessary for those reasons to count in favor of the belief, on the other. And in at least some cases, we should think that experiences can play that latter role in a way consistent with apriority. I'll close with an example, and continue with further thoughts and possible applications in another post.
Consider a moderately complicated proof. Suppose it requires a couple dozen lines, and involves fairly lengthy sentences. In fact, it is valid, and indeed, I have produced it correctly -- every step followed from the previous one in a way that I appreciated while writing it down on my blackboard. I've reached my conclusion, but I do not, at this moment, know it to be true. The reason this is so is that the proof is too complicated for me to know it sound straightaway; the chance of making a mistake is too great. Sometimes I apply rules incorrectly; sometimes I accidentally change variables. I don't do this very often -- no more than most philosophers -- but I do it often enough that it would be unreasonable to be confident in the soundness at this point. Instead, I should go back and check my work. I review each step, looking for mistakes, and find that I made none. Now I know the conclusion.
My experience of checking my proof played a significant role in my knowledge of its conclusion. Certainly, that latter knowledge at least counterfactually depends upon it. But these experiences, of course, weren't necessary for the possession of any concepts in the conclusion; I was perfectly capable of entertaining the thought before I began. And this is a positive dependence on experience; it's not just that I need to lack particular misleading experiences. Nevertheless, this looks like a merely enabling role for experience; my belief in the conclusion is not based on my experience of checking the proof; it is based only on the premises. If those are a priori, then my knowledge of the conclusion is too.
So there are ways for experience to play merely enabling roles beyond the ones articulated above.