As far as I know the MC7710 is the same as the MC7700; however, it supports different GSM/LTE cellular bands. WebOS contains native support for the MC7700 and it should be recognized when the kernel starts up. You can see this in the kernel log (dmesg). WebOS logs are located in under /var/log/. Look for a compressed file with the name dmesg.txt.gz.
I have attached an excerpt from the WebOS dmesg log for a TP 4G with an MC7700 installed. In this log you can see the isp1763 usb hub (usb port connected to the cellular modem) get powered on and start up, as well as the sierra drivers being loaded. Later in the log you can see 'usb 1-1.2: new high speed USB device using isp1763_usb and address 3' being recognized -- it is the Sierra MC7700 module. As you can see, it is started up and ports are assigned for various functions by the Sierra kernel drivers.
The Sierra kernel drivers do support both the MC7700 and MC7710. So, if your MC7710 is working right, you should see a similar sequence. Depending on various firmware settings, you may see a different set of ports being assigned.
I guess if you see the MC7710 is starting up fine, but it does not work right under WebOS, then it could be a WebOS support issue specific to the MC7710 vs the MC7700. Probably would work using my Sierra CM10 build.
When I first got my MC7700, I stuck it in a Dell laptop and it worked fine. Then I installed it in my TP 4G and WebOS recognized it right away. It connected to AT&T using the same SIM (however, the display only shows HSPA, this is a WebOS limitation).