I think it'd be a great idea to add an option in the admin to enable tinyMCE editing for the in-gallery description editing as well !

For now though, is there a way I can get this without editing the core?

tinyMCE is by default enabled for all descriptions fields in Zenphoto 1.2.4.

But it doesnt seem to be enabled for editing descriptions in the gallery view (not the admin panel).

That would really break any design if you do that. But it should be possible. You find all info how to add TinyMCE to text areas on the TinyMCE site.

Hi! I looked at the TinyMCE site for this information, but it does not explain how to add TinyMCE functionality within an editInPlace call... how would I do this?

BTW, I am trying to add TinyMCE functionality to the image descriptions editInPlace.

You will have to look at the imagedesc function on template-function.php and create a new tinymce config file that attaches it to that element.