Need ideas for HTTPS multiple domain solution

I'm developing a WordPress site that runs on multiple domains (not WordPress MU). The two domains are served from the same codebase and I have written a plugin that successfully switches themes based on the URL.

This site has a donation form that uses HTTPS (currently using the WordPress HTTPS plugin). However, WordPress HTTPS only has one option for the domain.

I thought that I could maybe get away with replacing this line $ssl_host = rtrim($this->getSetting('ssl_host'), '/') . '/';

...with this $ssl_host = $_SERVER['SERVER_NAME'] . '/';

Unfortunately, it's not that easy. It seems to break the whole site. The site header (and consequently everything else) won't load properly.

Any ideas? Any help here would be much appreciated.