I thought the community is here to support those people who can't afford to pay for staff support. There isn't any reason why premium members can't post here. You have the best of both worlds because you're supposed to have better support which is one of the reasons to get a premium account.
But to want to limit your community to only those who pay for premium? Seems a bit self defeating unless it's about the narrow subject of premium features not available to free accounts.
If it is about that narrow subject, posting in the support forum should yield results within 24 hours. Or you can get support directly through whatever method premium accounts offer (I can't comment because I've never seen them).
If it isn't about the narrow subject of premium features then I fail to see what it could achieve. People who have free accounts are just as adept at designing html as any other user. Paying doesn't make you auto-qualified and staff regularly check the support forum so if you want a staff response it's there. Maybe if you mention you have a premium account you'll get a quicker response. Needs to be in the support forum though.