Hi, I'm interested in joining this game jam, but the only experience I have with NES programming involves NES Maker which I believe uses mapper 30. Would it be okay to use this tool despite the fact the rules state mapper 28 is required?

Probably not for the "Category 1 - Game" group. You could ask around the NES Maker groups to see if anyone has ideas on how to fit it into the constrains of the competition though (see "overview" for more details).

However, anything that runs on the NES can be submitted for "Category 2 - Tools and Toys"!

There is also the up coming ByteOff3, specifically for the NESMaker community, so check that out for sure if you haven't already!