Can't tell you anything in particular about the BT7 and its compatibility with a Squier, but I can tell you that the Extender works very well. I have one on all of my basses, and I use them regularly. The Extender is a good design that is constructed robustly.
Be sure and read the tuning instructions that come with the key. If you tune the E to pitch, then drop the lever and tune the D, when you come back up to E it's commonly a few cents sharp. The trick is to do this, note how sharp it is, re-tune the E the same amount FLAT, then tune the D, then when you come back up to E, it should be in tune.
(Substitute your own values for "E" and "D" above.)
-t