Sorry, but if it was a BMT tunnel under Beford Ave. then it would be above the G and not below. Just about all NY's subway lines are built as close to the surface as possible.
If this was a BMT line it would have to have been built no later than the early 1920s (pre Mayor Hylan), when nearly all BMT & IRT subway construction stopped. There are no plans for this in any subway proposal I have seen - the BMT's (actually BRT's) proposed Brooklyn-Queens "cross-town" line was to be an el, and it would have been north of Myrtle Ave. (the G is south of Myrtle at this point).
Now there WAS proposals to continue the IND East from the G's Beford Station (it has 3 tracks), to completely replace the old Lafayette St. El, but that would put any unused tunnel under Lafayette and not Bedford (they intersect). This proposal did not make it to either the Board to Transportations new route proposals for either 1929 or 1939.
I feel this post is kind of foolish - you know you are going to a hidden place and you don't bring a camera?!?!?! What was Steve K. thinking?