You can supplement BDAG with Chamberlain's recently shipped The Greek of the Septuagint: A Supplemental Lexicon with the "Multiple Resources" display. Add the Supplemental Lexicon to BDAG for "Multiple Resource" display. Then, when you click a New Testament word not in Chamberlain all you will see is BDAG. But if you click a word that is in the LXX, or that has a sense additional to that in BDAG, it will come up beside BDAG.
Please let me know if you have found an even easier way to link Chamberlain to BDAG.