In every congo I've attended the elders are very sensitive towards making sure no one partakes "unworthily". If someone who is 1.) unbaptized, 2.) borderline or full-blown apostate or 3.) perhaps suffering from mental illness, partakes, they won't be counted from what I've seen.
The only way this idea would work is if someone showed up from out of town and talked a VERY convincing line of BS establishing themselves as a solid JW who HAD to attend outside their congo due to unforseen circumstances. Even then, there's a fair chance the elders would ask for contact info and check up on them before counting them as a partaker.
I think the only way this idea could work is if currently attending JWs start partaking in their local congos and swear to the elders that they really were "called" since the "new light" on 1935 came out last year. Personally, I'm not up for it.
OM