Cursify parameter?

Is the Cursify transformation filter available as a custom parameter for instances? If so, what is the syntax to invoke it?
Is there documentation of filter parameter syntax anywhere?

I don’t recall that filter being available in the Custom Parameters section. The syntax for the Filter custom parameter and list of available filters, as of May 2013, are on page 88 (Section 12.2 Custom Parameters) of the Glyphs Handbook, available on this page:

Detail is also in Eric’s blog post on Custom Parameters:

Cursify is not available as custom parameter and we have no plans to implement it this way. We strongly recommend to keep an italic in a separate font file.