Hmm . . the problem with co-ordinating through a planner is:
1) It doesn't properly model the situation in which one country was in the position to issue orders to the other (Germany and Romania, for example).
Puppets (and colonies) automatically agree - or at least have a very good chance to. If a country is not a puppet or colony I don't see much justification (although a degree of leverage might be available - relationship changing chances, essentially).
2) Other countries will have their own plans.
Of course - but the question is whether they sign up to a joint venture.
3) Other countries will alter your plan.
I think the system would need to ban this, once the plan is set in motion, for any but the plan's author. Adding tweaks to as-yet-unagreed plans is almost essential, but once the plan is agreed it is under military command.
Each plan will, it seems to me, need to be assigned a single, overall HQ in any case. The level of this HQ should depend on the scale of the plan, but split command is not something most militaries would countenance. Units that are agreed for the plan would need to get assigned to that HQ - becoming "expeditionary forces", in effect. The owner of the overall HQ is the only one able to change it once the preparation and execution phases have begun.
4) Like you say, getting them to even get to the start-point may not be easy.
Welcome to the politics of alliances! The Normandy invasions didn't happen on a whim Churchill* woke up with one morning!
*: Or Eisenhower, or anyone else for that matter. Agreement of the target of the plan, agreement of the SHAEF staff and control, agreement of committed forces and so on and so on all had to be agreed beforehand. It took years.
5) They will simply follow their own objectives once the plan is finished.
They might. Withdrawal of forces from the combined HQ should certainly be possible after a certain time. But maintaining a successful combined force (under the control of whoever owns the Supreme Operational Headquarters) could be possible, too.
6) It lacks the surety of direct control - HOI AI is infamous for pointlessly shuffling its units, and in a situation where you really have to have a province covered, losing that province because the AI just decided to leave it for no reason could be intensely annoying.
It looks like the AI will be executing all plans, anyway. At the lowest level, that's no different to earlier HoIs - you tell the program/AI to move unit B from province A to province C and leave the computer to get on with it - you dont have to click-and-drag the unit every pixel of the way.
In a combined plan, I guess this would/should work the same way. The plan can be as detailed or as outline as you like.
HOI2 had the right idea by simply allowing you to take over a country's military. A more limited version of this is the way to go.
OK, but that has a few problems:
1) It means your only choice is to manage ALL an allies' actions - including the defensive and routine elements - rather than just the important combined operations.
2) It does not really suit combined operations in multi-player (where you are playing allies).
3) It allows cheesy exploits of sacrificing your allies in order to obtain your own objectives.
Broadly, what work in the game as "expeditionary forces" were very, very common between allies. The entire New Zealand navy existed as a fixed "expeditionary force" in the UK Royal Navy up until 1942. The core of what is needed is the ability to request/demand expeditionary forces of your allies. The battle plan mechanism simply adds some context and political reasoning around that.