This currently is not possible, you can't ping an interface on a PIX from another interface, even over a VPN tunnel. I agree that this functionality is very useful in your situation where you have remote PIX's, and the only way you have to connect to them is via their inside interface. This is especially necessary when the PIX gets it's outside IP address via DHCP from the ISP, in that scenario you have no way of knowing if the PIX is up or not.
I believe this functionality is being included in 6.3 code due out late this month/early April, since a lot of customers complained about it.