As long as it doesn't adversely impact on the way the PIC is configured and run, you could use any Register as storage or as a variable.

I remember an old long retired IT Manager telling me when as a 'New Boy' starting as a junior, he spent a WEEK looking for a spare BIT that could be used in the company's IBM Mainframe Payroll Program which had filled all of the available 8K Memory! My, how times have changed!