First of all, the BMR was initially proposed in Rogaway's thesis in [1], whose extended abstract you mention [2]. There, the protocol was proven secure under active adaptive adversaries corrupting and controlling strictly less than half of the parties (e.g. $t < n/2$).
The original proposed BMR protocol has appeared to have flaws in it's security even against a passive adversary. For more information you can check this [3] paper.
Now, considering that the above problem is solved and trying to answer your questions, after a lot of research I couldn't find any clear formal proof that the protocol is passively secure against adversaries controlling $t<n$ corrupted parties. However on p. 95 of Rogaway's thesis the following statement is made :
REQUIREMENT FOR HONEST MAJORITY. The assumption that $t < \tfrac{n}{2}$ was only required
because Theorem 4.1.1 requires this; a garbled program reveals no useful information as
long as $t<n$. Indeed, the proof we have given remains unmofied to establish this claim,
apart from relaxing the bound in the conclusion of Claim 6 to $\ge \tfrac{ν(k)}{n} \ge \tfrac{ν(k)}{k^{10}}$
Also in [3] they prove that after their modifications their protocol is passively secure and they also mention the following :
Theorem 4.1 The circuit construction from BMR, modified with the use of splitter gates as de-
scribed in this paper so that the fanout of all gates is at most one, is secure against a passive,
honest-but-curious adversary.
From the above I assume that the protocol is secure against passive adversaries corrupting up to $t<n$ parties.
P.S. This paper [4] provided the best explanation of the protocol for me.