I only have EPD and MMP and both have X in Box 16. They both say " all of our income is effectively connected to a US trade or business", so my feeling is that there are no foreign taxes.
With the current fighting in Congress with those wanting to cut the IRS budget ( idiots or worse ) and those who believe they do have enough or we can't afford more, I really doubt the IRS is going down the K2 K3 path. My BIL paper filed his taxes last February
and got his refund this week, and the IRS is pulling staff from everywhere to try and
clear up the backlog of returns (amended as well). I will uncheck Box 16 and file.
The IRS has not decided yet, and that means the MLP's do not know how to submit the info, nor do the Software companies have clue. What are chances this will be all worked out in next 6 weeks or so.