Unfortunately this is not possible as recursion is limited by stack space. Stack space is limited memory that is defined for an application at compile time. Stack space is automatically used whenever a function is called, so recursion depth is always limited. If an application uses more than its allocated stack space, this causes a stack overflow crash. Most languages place limits on recursion depth because of this. In languages where you can increase the recursion depth, this can still cause an application/script to crash if it goes beyond the stack space limit. That is why the /maxdepth command was removed from mIRC and the maximum depth was set low.
The default stack space for Windows applications is 1MB. mIRC sets its own stack space at 2MB, which would technically allow recursion beyond 255, perhaps to about 1500, but it is set to 255 as a precaution as other routines can be chained recursively during script parsing.
Even if I did increase mIRC's reserved stack space, say to 32MB (which is not recommended), in my tests this still only allowed a recursive depth of about 30000 and used 600MB of memory. I could probably improve on these numbers by rewriting parts of the script parser but not by much.
There is a special case recursion call that conserves stack space called a tail call but this would not work for the scripter parser as a tail call requires a very specific set of conditions to work.
