I want to send/receive tells in instances.

If someone does not, they are free to filter them out from their chatlog. That function is already in game.
Every modern MMO allows chat in dungeons and there is a reason for that (hint: doing otherwise is stupid).
You all are mistaking your fear of RMT tells with the fear SE would be too incompetent to handle RMT tells.
Right Click->Report spam-> 10 reports from ten different people => /tell and /shout disabled for 10min and case submitted to GM review
Other companies have figured it out already, I'm sure SE can in not too distant future as well.