I would think it'd be fine, alternatively you can cross post it if you want to post it to some contest community if something like that exists.

Given that it rewards GRC I think it's more than enough to count as GRC related, It's also Crypto related as well and I'd welcome the extra activity on the community.
Also it's a once a week post also with makes it even more fine.

As for an api, I'm not sure.
What you can do is set the first category as hive-161364 (which is the gricoin community account) it should make all the front ends link up to the community automatically as if you posted directly to the community.

I just tested this on steempeak posting to my blog but with the tag hive-161364 and it showed up in the community. At the same time you can add Gridcoin so it's on the gridcoin topic as well along with any other tags you use.