Theming HTMLMail: Alternative page.tpl.php

I'd like to know if it is possible to use an alternative page.tpl.php for emails sent with HTMLMail? I tried page-htmlmail.tpl.php, but it doesn't work. I'd like to reuse most parts of my theme (based on garland) but not all, so I want to use a slimmer version of my page.tpl.php for sending emails. I know I can use an alternative theme or a subtheme as workaround, but it's more unnecessary work.

I think, this might be a nice feature, if itsn't already existing.

Thanks for this nice module, regards
Helmut