Ok, but what if they don't kick?
That's what we have in NCAA, and they don't have to kick. But, once the SKF is established, numbering exceptions are all - and always - ineligible - just as though they are wearing numbers 50-79. We only need to know the eligible folks at the moment the SKF is established - the two ends wearing eligible numbers, and any backs wearing eligible numbers. Wings can get their end and nearest back. R can get the holder and kicker. When the exception(s) is(are) declared, they are locked into being on the line and between the ends at the snap. They can't become an end, or a back, regardless of what shifting they do, or what final formation they assume. If an 'exception' moves to an end or back position, and is there at the snap, you'll at least have an illegal formation foul. If anybody other than the 6 possible eligibles touches a legal forward pass (LFP) before it is touched by B, or are downfield (by whatever criteria you use) before a LFP that crosses the NZ is thrown, you've got a foul. If they shift out of the SKF, then numbering exceptions are not permitted, so there ain't much incentive to do that. Just be aware of who (by number) touches a LFP, and/or where. If it ain't one of the 6 possible eligibles when the SKF was established, then you've got a foul. And, if someone other than the 6 possible eligibles is downfield before the LFP is released, you've got a foul.
All of this is intended to keep Team A from unfairly confusing Team B as to which players are eligible. Like I said, the best way to look at it is: as soon as the SKF is first established, any 'exceptions' are now, effectively, wearing numbers 50-79, and they have to comply with rules related to player numbering - and that can't change, unless there is a time out.