You could use transparent HTTP monitors.
Transparent
Specifies whether the monitor operates in transparent mode. A monitor in transparent mode uses a path through the associated pool members or nodes to monitor the aliased destination (that is, it monitors the Alias Address-Alias Service Port combination specified in the monitor). The default is No.
Yes: Specifies that the monitor operates in transparent mode.
No: Specifies that the monitor does not operate in transparent mode.
I've done something similar with default gateway pools. If you want to do an HTTP check for google for instance, you'll type in their address in the "Alias Address" field and port 80 in the "Service port field". Then simply attach that monitor to your pool member/nodes. Since google obviously has multiple IPs, I'd recommend configuring several monitors and requiring at least X be available.