There is a ban on running using another competitor’s number in all disciplines. Without asking people for ID when they collect their number, this is unenforceable (the only race I’ve been required to produce ID for is London Marathon). I’m not suggesting that fell race organisers should ask all runners to show ID when collecting numbers as this would be wholly impractical given the reliance on a small number of volunteers in ratio to runners. However, the ban on swapping numbers is upheld in spirit - there have been instances of DQ & penalties against clubs in road & XC races I’ve known of.
Similarly, a ban on GPS for nav can still be upheld, even if it appears unenforceable (without someone speaking up against those they see using it). Appealing to the good nature of runners is part of our sport - just as much as asking them not to climb that wall/cross that field etc. is.
I’m with Nic - leave it to the RO. Plenty of us don’t have a watch with nav functions, so I cannot see it splitting the sport.