Unfortunately, the Xyce parser by default doesn't allow any expressions in
analysis statements such as .TRAN (or .DC, .AC, etc). There are some
exceptions to this, but that is the default behavior. So what you're trying to do isn't currently allowed in Xyce.
This has come up multiple times before (at least once in this forum) and is an issue we need to fix. A version of this is in our internal issue tracker already, but we haven't gotten around to resolving it.
The code was probably set up this way (I am guessing) to prevent expressions that contain changeable parameters; i.e. params that could be updated via .STEP. The various analysis classes at the moment aren't smart enough to update stuff like this once the simulation has started. Fixing the code to handle this use case in general will require some hacking on the setup functions for each analysis class. That isn't impossible to do, but will take a little time.
On the other hand, the use case of expressions that *don't* change should be pretty easy to fix, as this is mostly about revising some logic (I think) in one function in the parser. I'll see if I can make that happen quickly.