I believe 10 or 15 years ago Toronto initially converted some parking meters to bike "post and ring" stands. However I think now the post and ring stands are separately manufactured. Not sure who you would ask...try the city cycling committee. http://www.toronto.ca/cycling/index.htm
T-bar welded to the top, ring like the old hitching posts bolted on with tamperproof bolts. disadvantage is that it would take equipment at the pole and increae costs.
If real low-buck is the option, use pipe caps with a ring or T-bar maybe both welded to them then installed with epoxy, unless you have different meter mountings, all the ones I've seen were thread-on types. ring or T-bar would make it impossible for someone to lift a U lock over the top if they were made of a sturdy enough material, the epoxy would prevent unscrewing the cap. advantage is that all work can be done inside and the only thing you need at the pole is a 'wrench' and some epoxy
I know about those and they are too expensive. We are thinking of just some sort of cap so U locks would still work.
Expensive is a relative term. A bar on top of a parking meter post might injure some child running and lead to a lawsuit. A ring would be less dangerous. Also a bar or cap wouldn't help people with cable locks that might be long enough to slip over the top, but some people might be inattentive enough to lock up that way. I suspect my last stolen bike was due to a similar error - I might have had a lapse in attention and locked my cable to the end post of a rack with a T-shaped foot that they lifted and worked the cable around. Some kind of closed ring or triangle or whatever is the best thing to lock to.
Weld a solid bar vertically to the post so that it is attached at the top and bottom with about a 4" gap between the post and the bar. Anything locked around the post, between the post and bar, couldn't be lifted over the top, or worked out from under the bottom then lifted over.
A hole drilled in the post would work, but only at a fixed height. A bar welded on vertically would make almost the entire height of the post usable.
The T-bar idea would work, but the city might be concerned about liability.
You'd have to use the tool on their website to request quote. It must depend on how many you order and probably some options.
I don't know at what level you're involved in your community, but you should talk with your city council and engineers to have something made on-purpose, if that's possible and/or not cost prohibitive.