"Comcast says that non-subscribers should search for the xfinitywifi network name and click on the Not a Comcast subscriber? link at the bottom of the Sign In page and then select the Complimentary Trial Session option from the drop down list. The free hotspots will be available through November 7th."
»bgr.com/2012/10/31/comcast-hurri ··· otspots/
I don't know if this helps or if you have tried it yet. May be a shot in the dark.
EDIT: I haven't used DD-WRT in a while but I do remember being able to set it up as Repeater Mode. That is untimately what you would want to do. Receive signal at Gmas and push it to Moms. If I understand you correctly. Comcast may block this some how to prevent abuse but beings they are giving it away to non customers I can't imagine why they would or how for that matter.