(*) I'm not sure if any (solo) pool will support low hash rate miners in the future. Pool operators have to pay for their hardware and connection, and a low hash rate miner costs them as much as a complete mining farm. Maybe CK can answer that.
I think 0.2th - 0.25th is and will be for a good period of time enough hashrate to reasonably connect to any mining pool, it's completely fine if I get a valid share every 5 mins instead of every 5 seconds, the pool manages its resources through share difficulty and the hashrate of the device is irrelevant to them, they maintain certain traffic between their server and your miner/proxy by raising/lowering the difficulty.
Now the real question is, what is the price tag? I know you said you don't know, for you to get answers you need to find the answer to that question first, i might be interested in buying a few if it was $50, maybe buy 1-2 if it was $100, ditch the idea altogether if it was $200, so ya, sadly, it all boils down to the price.
Well, the pool needs to send new work every 30s/60s, whether you are mining with 1MH/s or 100PH/s..
The few bytes that get send back from the miner are irrelevant (also I've limited the max. tickets in my software to 3 per minute, but if you compile from source you could remove that limit).
Just think what 1000 of these miners will do for damage, or 10000?
I'm thinking of also creating a simple proxy (also on an ESP) so the pool would only have to supply work to one connection instead of having to waste resources on multiple miners on the same IP address.