db_url.connect does not support pooling
In the mean time, for anyone who comes across this problem in the future, I was able to get a temporary monkeypatch working;

``` py
from playhouse import db_url
from playhouse.pool import PooledMySQLDatabase
db_url.schemes['mysql'] = PooledMySQLDatabase
```

The above will replace the MySQL backend with a pooled backend, whenever `connect()` is used. Replace classes accordingly for other databases, see [here](1).
 You can use `db_url.parse(url)` to parse the data into a dictionary, then pass the dictionary values into a pooled database implementation. Because pooled databases require additional arguments (pool size, etc), they do not seem to me a good candidate for inclusion in the db_url module.
 That works beautifully, thank you.

As seen [here](1), there is no way to make `connect()` use a pooled connection. I've looked over the code and there doesn't appear to be any way to convert an existing DB object into a pooled one, at least short of using some horrible metaclass hacks or monkey patching.

Could someone clarify if this feature is on the roadmap, or whether a PR would be accepted if not?
