It just seems a bit "slippery slope": first we got :scm as a hash map, then maybe add :licenses as a vector of hash maps, then some other options for other XML fragments folks think are important to add... It's going to make for an entire DSL just in options!
There has to be a better way. I think adding :scm was a mistake, TBH.
How about an inline template as a single hiccup-like structure? Just one option, it's data, it can support _any_ pieces of the XML file that folks want to generate...