[smartMart]

Originally Posted by Vergil

However, what I am concerned about is if it is possible to insert my own HTML elements to the storefront using things like for instance hooks (as in wordpress) or smarty blocks etc...

You can insert your own HTML elements to the storefront widget using custom JS.

Originally Posted by Vergil

so that even if the user changes Shop Themes my custom div will still be positioned in a specific location (i.e.product page)

In general, YES.

BUT: advanced (and malicious) user can hide/move/"replace" virtually any item on storefront using just stylesheet (CSS) rules. The same applies to elements inserted via wordpress hooks. So just keep it in mind.