That's exactly what I have been trying to do for testing:
Code:
header set x-test-header true
OLS sends it normally, if I use the "Headers" field for a "Context" (besides an issue with "merge", that I've reported already), but it seems to completely ignore the directive in any field related to rewrite or respective .htaccess. And, as mentioned, I can confirm, that it is reading the .htaccess, because for the test sake I moved some of the writes from VHost settings to the file.