Thanks @Eduard Lebedyuk ! I'll give it a try!

Will it work for POST requests too?

And is it for Postman only? E.g. if I use swagger-ui how could I get this page displayed?

Another one-liner suggested by @Robert Cemper recently:
 

k ^SPOOL s %io=$I O 2 u 2 ZW %anyvariable c 2 u %io

E.g. if you want to expose the content of %request during REST API method debugging, to write all the contents of %request to ^SPOOL global:

k ^SPOOL s %io=$I O 2 u 2 ZW %request c 2 u %io

and then ZW ^SPOOL in the terminal.

It is a global, again, though... But a very interesting and useful way to convert any write to a device into a global. 

Deserves a one liner input!