computer says no
RMC,
I have a feeling that being a reservist is one of those not permissible by NATS for reasons Alosaurus has already mentioned. I also think bring a retained fireman or a bouncer are also on the no list. The fireman probably because you can't abandon your position for a 'shout' not sure on the second.
Personally, I wouldn't stop or declare your RM application until it becomes a factor with NATS.
Sly