You can submit a song that doesn't use expansion audio at all to an NSF+, it's a superset of the NSF format on this site.
sure, you'd be at a disadvantage going up against entries using expansions, but it's otherwise fine
as for 7bit PCM -- that's ostensibly allowed in even in normal NSF compos unless explicitly disallowed by the host -- it's a trick you can do without any additional hardware on the cart, driven in software, and I've used SuperNSF to produce NSF music with PCM channels before on this site (up to 4 software mixed channels, although with all 4 enabled, it sounds utterly awful)
take note that none of this is strictly binding despite the admin logo next to my name, but you will certainly be fine submitting an entry without expansion audio to the NSF+ format :Y