If you really take a hard look at first and second editions they are close enough to be the same game. I Know it hurts me to say that, as 2nd edition is an abomination that should be burned, but its true. Third and Three point five, even pathfinder could be one system. Fourth edition is its own ballpark. The only things that are common amongst all systems are attributes and hit points. And in truth the scale of those varies.
With players that are devoted to a particular system they are going to want to play entirely in that system. I have been at enough combined first and second edition tables to know the players want to handle themselves entirely within that particular rule set. The game will need to treat each player as a black box. Input going in and out will need to be in some edition specific language or a language that player can be convert them self. Everyone at the table will need to convert fifth edition to their own edition language which gets complicated.
Your first and second editions players are going to give answers like yes and no to saving throw roles. The rest will be giving the DM saving throw values. This could be handled simply enough by just accepting the yes and no as the saves are more granular in lower editions. I mean they have five or six saves for rods and poison all sorts of stuff in the early editions of the game. Perhaps this allows for automatic saves that are now allowed in other versions of the game but so what. To hit roles will be a little more complicated with third and fourth edition players saying they rolled a 35 while the fist and second edition players say they hit a thaco of -10 or -15. You will probably need some sort of damage increase between game versions as well.
One of the biggest problems I see are skills. They don't exist in first edition. Skills come in as either having a skill or not in second edition. In third and three point five skills are core to the game. In Fourth skills are there but with very limited control over where points are placed. This creates a problems where first edition folks are just going to need to role play out ideas with other edition players making dice rolls. A strong DM will allow this to flow the same with the third edition folks influencing their skill roles with role playing ideas and what not. The problem would be DM's that encourage your ideas but having the rules coming down to a dice roll. These guys will derail the train.
One of the problems here is giving a first edition player skills means they are not playing first edition. Any modification to fit a common system is going to effectively break their edition specific gaming experience. Items may need to be multi stated in this light. Will player be able to over rule special abilities that are not present in their version of the Dungeons and Dragons system? Early editions don't allow for much increase of attributes but this becomes common in later editions, even statistically required.
I am having difficulty finding much more information other than the fifth edition is being worked on. If you have a site that has some better details please let me know.