This is a rehash of an idea posted here:
http://forums.alliedmods.net/showthread.php?t=120710
Which would be that a player could choose to be a Scout and then the Scout has a partner Demoman. The Scout has all weapons disabled (Optional to allow Bonk use).
They are sort of paired as teammates. Maybe have a panel open on the left when the Demoman/Scout are selected asking them if they wish to be the Bomb Supplier or the Suicide Scout. Could be a cvar to limit the number of Bomber/Scout pairs.
So if you set the limit to 1 you could only have one Demoman to act as the Bomber and one Suicide Scout.
As said above, Scout cannot use or fire any weapons (Except Bonk through use of an allow cvar)
The trick would be to prevent the Demoman from detonating his stickies unless his stickies were attached to the Scout. If that was not possible, strip the Demoman of all his other weapons and only let him use Stickies and maybe start him with only 8 stickies and not allow him to refill until he dies.
The idea is that the Scout and Demoman work together and the Demoman Stickies the Scout up who then can run into crowds of enemies to get detonated.