Custom dynamic selection variable for standard SAP reports

Hi. Is it possible to add a dynamic selection variable that can then be used by standard SAP reports?

For example I made an entry in TVARVC with "SY-UNAME" as the value, and that can be selected in ME2N, but it comes through as a literal, it does not read the system user name.

Is there some way of either getting system fields or perhaps specifying a function module to read a custom table for the values?

We want to use it on standard SAP reports so changing the declaration of the select options is not an option. We could create a Z report as a copy, but we would prefer not to.

Thanks a lot for your help,

Dave.