Hi, I have recently also bought a MK1 and have been thinking about some of the mods you are considering. I was also looking at an older MK1, maybe the one you bought (mine is 10 years old so no spring chicken!), and did some reading about the Milanes foils that the early boats were supplied with - apparenty pretty good and, having used Milanes foils on other classes, I wouldn't be rushing to change ithem (expensive and, for me at least a marginal gain at best, the Milanes profiles of that era are very good). Milanes do a reasonably priced refurb service although rebuilding the edges isn't that hard for a DIYer either (I have done it and can't claim any great skill!).
My boat has three holes drilled for continuous control lines and I'm thinking of using screw in contol line bullseyes if I add more as it will be simpler, and provide more options.
Like you, I am also looking at the inhaul. My plan is to add an additional cleat each side on the flat part where the cleats for the toe strap are. However, I think I'm going to mock up an inhaul control first and see what its effect is - I have focused on maximising the travel on the outhaul (added some elastic) and this has a massive effect. I have only owned my boat just over a fortnight so I haven't adjusted the cunningham or the kicker in anger so I am going to see what effect they have (I know what they do, I just want to see if I really need more adjustability - it might be better to just focus on sailing the boat) before getting too focused on the inhaul).
As a halfway house you could use a Clam Cleat Q-Lok which would allow you to adjust the inhaul but only between races. I appreciate that you may want to tweak it all the time so this approach won't be adequate.
I very much like the idea of adding centreboard controls as that is something I would like to be able to adjust on the fly from a hiked position - maybe someone can share a rigging diagram?
I would also like to see a rigging diagram for a continuous traveller system. The trend seems to be away from doing too much with the traveller but I would like to only have to set it on one tack and for that setting to be automatically replicated on the opposite tack.
Final thought. If you need to free up cleats, you can change your toe strap adjustment to use an inline Clam Cleat which can be attached from the thwart - my boat has a suitable spare hole on each side. The model is CL828-68AN. I don't adjust my toe straps that much, if at all, so I don;t really need to take up a cleat position.
Apologies for the very long response but I thought I'd share the fruits of my research. I haven't done any modifications yet as I have only sailed the boat twice (both times in Opens) and I think there are plenty of speed gains I can make from simply sailing better!
Please keep us posted here as I'd be interested to see what you end up doing - especially if you can share pix/diagram of your centreboard solution.