The next version of SchXslt2 will support a processing instruction schxslt.declare-param to aid writing ISO Schematron w/ abstract patterns. I chose a processing instruction over reusing the sch:param element to not interfere with the current standardisation process.
If the processing instruction is used, the transpiler terminates with an error if a declared parameter is not provided, and if a provided parameter ist not declared.
https://git.sr.ht/~dmaus/schxslt2/tree/main/item/src/main/resources/content/transpile.xsl#L186
And regarding https://github.com/Schematron/schematron-enhancement-proposals/issues/64: You already can use attribute value templates, next version of SchXslt2 provides a global variable schxslt:phase to the validation stylesheet.
...and while I am at it: Next version will also get a transpiler parameter
schxslt:expand-textthat enables text value template expansion in the validation stylesheet.https://git.sr.ht/~dmaus/schxslt2/commit/0f2672c76224ddb983c5c9d4a97157055e42f1e1
See also https://github.com/Schematron/schematron-enhancement-proposals/issues/49