Is this possible to do?
Yes
Open pppoe-out1 client, or other name if is not the same,
on Dial Out read what "Profile" is used, close this window without change anything.
now go on the PPP / Profiles
open the used profile on pppoe-client,
go on Scripts section and on "On Up" field put the follow command, DO NOT CHANGE THE COMMAND, it is ready as it is, (do not write anything on "On Down" field):
/interface 6to4 set [find] local-address=$"local-address"
Save with OK,
next time you reconnect the pppoe-client, the command update the IP on 6to4 tunnel.