Oh man, I think this might actually work. Feeling adventurous this evening, I decided to look into the XPage runtime's expression language handler. After poring through tons of methods, interfaces, implementation classes, EXTENDED implementation classes, and disparate JARs, I narrowed the prefix handler down to the "FactoryLookup" property of the IBM-specific variant of facesContext's Application. With that, which is basically a hash, you map a prefix to a handler factory (it's always factories with Java, isn't it?). Once there's a handler registered, you can then bind expressions with that prefix.

This actually means what it implies:

<xp:text value="#{ruby: 'hi from ruby'}" />

And it works! Better still, the show-stopper in Designer seems to have been resolved in one of the recent versions: rather than error'ing out when it sees an unrecognized EL prefix, it nags you with a warning but otherwise proceeds without problem, saving the script as-is in the resultant Java.

Now, this is VERY much a first pass at the idea - all I did was write extremely skeletal implementations of the Factory and Binding classes and then I register them in the beforePageLoad event of a page:

var app = facesContext.getApplication()
var facts = app.getFactoryLookup()

var rfac = new mtc.ruby.RubyBindingFactory()
facts.setFactory(rfac.getPrefix(), rfac)

I can't even begin to stress how not the right way this is. It doesn't work in all controls (repeats, for example, seem to do an extra syntax check), it doesn't yet have any context from the surrounding environment, and, most importantly, who knows what horrible things it's doing to the HTTP stack?

Still, the crucial point is that it really, really looks like it can work.

The two classes I wrote, which no one should, under any circumstances, use, are here: https://github.com/jesse-gallagher/Domino-One-Offs/tree/master/mcl/ruby