I'm no lawyer either, but to my knowledge GPL extensions are not allowed in PECL.
The reason is that PECL is a part of PHP project, which mostly uses Apache-like PHP License,
and GPL extensions would conflict with it.
I know this is the case with PEAR, PECL's PHP cousin. Looking at the account request form (http://pecl.php.net/account-request.php) it appears that PECL does NOT accept GPL extensions and, furthermore, PECL strongly discourages LGPL libraries. It appears the PHP License 3.0 is the preferred license for PECL.
That being said you have a few options:
1.) Ask the Pidgin team to give you the right to release the PECL extension under the PHP License 3.0 (I'm guessing either the Apache or New BSD would be acceptable as well).
2.) Release it under your own custom PEAR channel, which I *believe* will allow you to distribute PECL extensions without issue. Ask Greg Beaver for more information.
Despite the licensing issues I'd absolutely *love* to see an extension like this for the PHP community.
Thanks!
--Joe