It's possible - and is a PE/Compiler issue - but it would add confusion as non-X2's use the same pin numbers for input and output, so A.0 and B.0 may be ambiguous. Consider the following for 18X ...
HIGH B.0
SERIN A.0, N2400, b0
HIGH A.0
SERIN B.0, N2400, b0
b7 = A.0 ' or b7 = B.0
HIGH b7
SERIN b7, N2400, b0
Because A.0 and B.0 can be used interchangeably, it's very easy to imagine that any pin can be used for input or output as per X2's and I'm sure someone's going to do that then get very confused when the chip does something other than expected.
You can use "Symbol A.0 = 0" etc to add the port.number if you wish.