Two column align environment with one line spanning both columns

this is also a hack, but it will give the desired result.

use \rlap to "hide" the width of the overlong line. for the example given, i would do this for the last line:

L3 &= 1234567890\rlap{$abcdefghijklmnop \ .$}

you might even want to put some extra space after that to get the desired separation; just look at the output to see what's needed.

most lines of math contain reasonable places where one can break without damage.

Update:
In a comment, @user2478 has suggested using \mathrlap; requires mathtools. (Yes, this is better than plain \rlap.)