referencing/linking to an entityform and passing nid/other field information through the URL
I finally figured out how to get everything I wanted to happen, and figured I would post it here in case anyone had similar difficulties.  Entity reference prepopulate + a views block that links to my event registration entityform.  This blog was extremely helpful: https://wunder.io/blog/simple-contact-form-per-content-item-with-entityform/2012-10-24 . 

One tricky thing for me was getting my taxonomy terms to be recognized by an entity reference field.  The only field option I had was "Content: All taxonomy terms," and entity reference taxonomy term fields will only recognize the taxonomy id in the url.  So, I used views to rewrite the output of the  content: all taxonomy terms field as [term_node_tid-tid].  

Really effing love tokens, views, and entity reference prepopulate right now!