Is there an easy way to improve this Tree Normal Form for Busy Beaver candidates?
Your proposed optimization is a special case of the "forward reasoning" nontermination detection that Marxen and Buntrock describe in part 5 of their paper:


  If there is a set S of states such that all transitions from elements of S are defined and their target state is also in S, (and the machine is in one of those states) it will never again leave S and thus not halt (closed state/transition cluster).


If you've implemented this nontermination detection then your optimization is redundant.  You could include it for a possible slight efficiency win.  Your optimization applies whenever we are defining the 2k-th transition and there are currently only k states.  That will not be the case as often as you might think, since Marxen and Buntrock's enumeration algorithm does not assign transitions in order by state index.  Instead, it simulates each Turing machine during enumeration and assigns each transition only as it is reached.

The "forward reasoning" nontermination detection will catch cases that your optimization would not, e.g. in this partially defined 3-state machine:

    0    1
A: 1LB, -?-
B: 1LC, 1LC
C: 1RB, 1RB