Does anyone know of a way to script a feature in OpenType which would allow someone to typeset a carriage return and substitute the first lower case character typed with a substituted upstroke character for a script face?

I can get the following to work:

feature liga {
sub space a' by a.upstroke;
} liga;

But, interesting enough, I can put a unit value in FontLab glyph decimal slot 13 (CR), and script the following:

feature liga {
sub CR a' by a.upstroke;
} liga;

This will complie without errors, but will do nothing in InDesign when I typeset a carriage retun and typeset lower case a.
It will not substitute the upstroke lower case a.

Does anyone have any ideas.

Many thanks in advance.

Alex