Right now there is no option for that,
Battlegroups mode would be good for it but is buggy and as far as I remember starts lagging after a while.
Best solution for now would probably be a few spectators that use the final boss heirloom pack and make screens of people using heirlooms.
If necessary I can also create a pack that uses default textures for all non heirloomed items and makes all heirlooms be bright red or something.
Important part here would be trustworthy people watching it since it's pretty easy to change some files and have it look like people are using heirlooms although they are not.
I'll ask if there is an easy way to disable them completely but if it is something that takes more than a few minutes I doubt it will be added in the near future.
