>Additional flag to ngx_http_subrequest() can't help if you aren't
>even going to execute subrequests (as in filter finalization case)
>and can't help with ssi either, as subrequests are executed after
>main request already cleared request body.

>Maxim Dounin

Hm. I skipped this part. Sorry.

If it's flag in struct ngx_http_request_s() then you can control inside your module who will do the cleaning stuff, right?
So that you can set it to main request or to sub-request.