That would mean I can do whatever I want as long as I provide a way to invoke their unmodified interpreter rather than my hacked-up one (i.e., I could build two binaries). My reading of the license (modifying the software is only acceptable for one of three purposes in bold, and "not open source" appears verbatim) doesn't make me confident they'd actually be okay with that, though it'd be a pleasant surprise.
My understanding is that would be fine so long as the default distributable will run Shen according to the spec. Keep in mind, you can do a LOT of extensions and still run standard Shen. For example, the existing shen-clisp, you can seamlessly call clisp functions by typing them in all caps. I think it would be interesting for someone in this conversation to suggest something that they might want to do to the language that would not allow standard Shen to run also.
