Open tuning would make it a lot easier. The main problem is that once you tune the guitar to open D most of your songs would have been in the key of D. Now using a capo will enable you to go to different keys without retuning your guitar. If you're wanting to do some blues or rock in D the second position for the harp will be using a harp in A.As you move the capo up or down the neck the tuning of the guitar will change and, so will the key of the harp also. Hope this helps.
Hi Ed, I know this is a little old, but here is a tool to help with learning to tongue block, that allows you to see what your tongue is doing.