Got it. Yes, it looks like I misunderstood you.
What are you trying to achieve?
May be you can simply create parameter <*:Envelope* wildcard parameter and disallow some metacharacters for parameter name?
As I see, the only way how you can do it, is to create explicit parameter with "Dynamic parameter name" type, but this parameter can be defined only with Flow parameter level. So, if you have strictly defined flow (path from URL1 -> to URL2) and this parameter must be actual for it only, then you can use such approach
Thanks, Ivan