Good question. I just got a jpac with my astro cab, and tried hooking up the DC-VGA output through it with bad results. The jpac has an onboard video amp, and the DC-VGA is already amplified, so what you end up with is your screen fading to white as the signal gets overloaded. Thus you can't switch the jpac's vga input with computer/dreamcast (its not going to work).
The only solution I can think of would have to then be using a switchbox to switch the monitor from Jamma to Dreamcast. This means creating a custom vga cable for your arcade monitor that goes into the switchbox's single output port, and then run the DC-VGA output to one of the switchboxes inputs. The other switchbox input would have to connect to your jamma-RGBHV cable. You'd have to make this custom cable as well.