I have not done this myself so can not speak from experience. But I would think that it is possible to get this to work. You would do the regular configuration of site to site VPN on your 881. It will generate the IPSec packet with its private address as the source and at the external 3G that address will be translated. If you can determine what address the 3G is translating to you may be able to use that address when you configure the other router for its peer address. But since it is possible that the 3G might not always translate to the same address it is likely that on the other router you may need to configure a wild card for the peer.
One implication of doing this is that your 881 will always need to be the one to initiate the VPN tunnel. Since the other router will not be sure what your address will be it could respond but not initiate the tunnel.
This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
j ai une question j ai achete un routeur cisco 887VA-k9 , je le configuré avec la configuration ci- dessous
si je le lier avec mon pc portable sur l un de ses ports directement ça marche toute est bien ( la connexion internet + m...
Attached policy provides CLI access to the Cisco 4G router over text messaging. Two files are in the attached .tar file:
2. PDF with instructions on how to load and use the .tcl file.