Software handshaking (XON/XOFF) is not supported when using the RAW protocol for HP PCL because there are some PCL commands that require all 8-bits of ASCII to represent the command. Thus, RAW mode allows for use of all 8-bits as data (no filtering of any flow control characters) and therefore hardware handshaking is required. This holds true for all printers, including HP printers, when in PCL RAW mode.
In terms of a resolution to the problem, you can try using switch position 2 on the LaserWriter Pro 630 which places it in PostScript mode, 19200 baud, with XON/XOFF. Then you can send the line of PostScript below when you want to place the printer into HP emulation (only takes effect for the current job). This setup should might meet your need of having software handshaking in nonðbinary PCL mode.
PostScript to switch to PCL emulation
currentfile /LaserJetIIP statusdict /emulate get exec