I just revisited my old favourite Stormbringer, and found that in that game there's a neat mechanic for how to handle reach and speed of longer weapons. As you probably already know, there are rules for weapon speed in AD&D, but they are fiddly. This is how I'd adopt the Stormbringer rules for D&D.
These rules are hereby designated as Open Game Content via the
Open Game Licence.
Long weapons and Reach
Weapons are rated as Long or Short. If you have a Long weapon, and are not Engaged, you will have the advantage and may attack once before any enemy with the same initiative count.
To Engage an enemy you roll 1d20 below your DEX, forfeiting an attack to do so. After that you will have Engaged your enemy, and that figure loose the advantage of having a Long weapon, and must Disengage to regain that advantage. Disengaging uses the same procedure.
Optional: If you fail your roll to Engage/Disengage, the opposing figure get one free attack at your figure.
Group initiative
Roll initiative as usual and use the rules as written, except that before the first action are taken by the side winning, all figures equipped with a Long weapon get one strike each. These attacks are resolved in DEX order, and then the initiative proceeds as usual. No figure may attack more than once per turn in this way, unless explicitly allowed by other rules for multiple attacks.
Individual initiative
Roll initiative as usual. When resolving actions, first make one pass through the count down for all figures equipped with a Long weapon. Each may may one attack, in DEX order. Then go through the count of actions once more for those who have yet not taken an action. No figure may attack more than once per turn in this way, unless explicitly allowed by other rules for multiple attacks.
Long time readers of this blog might remember that I wrote about this rule once
before. That rule used the initiative roll in stead of a DEX check. It would be fun to try both out and see how they feel in comparison.