So what is OP_EVAL?
It's a way of letting you send to addresses that have more complicated restrictions on how the coins can then be spent - such as requiring signatures from multiple people or devices - without the sender having to worry about what the exact requirements are. It also makes it easier to, for example, set up an escrow arrangement where some coins can only be transferred either with the agreement of either the buyer and seller, or by the escrow agent and one of the two parties agreeing - so the escrow provider can't just run off with them. Sadly the tools to actually make this kind of escrow possible aren't quite ready yet.
There's a
good technical description here. It was originally going into Bitcoin, but I think they've pretty much abandoned it in favour of something similar which may or may not get enough support. It's all a tad messy on their end right now.
Do you plan to add support for Open Transactions?
Not really. That'd probably best be done externally by someone with more experience with Open Transactions, if at all.