As far as the SAF goes, JWs are only a problem if they refuse to
do military service. Ditto any other conscientious objectors, on
whatever basis.
Will the online admittance be forwarded to a Govt blacklist?
Probably so, but I always thought they only legally charged anyone
who owns or distribute the literature, or holds a meeting. (They
can also dismiss any practitioner in their employ who refuses to
sing the national anthem or salute the flag.) Otherwise, they're
"tolerated" as a underground religion, i.e. no direct harassment or
persecution.