Sorry for the late reply -- ubuntuforums was first down, then extremely slow, when I first tried to reply.
Thanks, I'll look at that the next time the problem occurs.
The situation actually resolved itself -- magically -- later in the day. That's a bit uncomfortable, since I don't know why it broke and I can't make a plan to fix it if it happens again (other than look at AP isolation).
A musician friend suggested connecting the mobile directly to the computer's wifi card (Atheros AR9285). I looked into that, since it would save me the trouble of carrying a wifi router to gigs. I had some success at first with a hostapd-based solution, but then the dhcp side of it stopped working (mobile couldn't get an ip address) and I haven't had a spare second to look at it since then. Also the internet sharing failed, despite the script  running
Still playing with it... but at least, if I can get the dhcp side working again, then the mobile's UDP messages do get through to my lappy.
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -s 10.10.0.0/16 -o \$ext_interface -j MASQUERADE