"John Clonts" wrote in message
...

He would need two flight plans for this, right? One to get up, one to get
back down.


Yup.



If instead, he requested VFR-on-top, do you think he could generally get
the routing he wanted? I mean what would ATC generally do with a request
like "N123 request VFR-on-top 7500 direct XYZ VOR, otherwise I'd like to
cancel IFR and request VFR advisories".


Assuming the controller isn't baffled by the request, and he's not heading
somewhere a preferential routing applies, he should be told to climb and
maintain VFR-on-top and report reaching it and also be cleared direct XYZ
VOR.