Turns out, we do have a way to do this, it's just not well-documented yet. We'll try to fix that this week, but here is some information.
Pick the advanced webhook when you're setting up your routing rule.
This webhook types accepts special variables in the URL or the payload. You can use
<<decdata>> in either of these to insert the data of your request into one of those.
For example, if you want to send a POST with your data in the URL, you can do something like this in in the URL field:
This also works in the payload field. To completely leave the payload out, put
_NONE_ in the payload field.