Another option is to put the continuation marker at the start of the next line, and allow it to take effect across multiple preceding comment-only or whitespace-only lines. Eg:

OBJECTS = one.o
\\          two.o # This object is notable for some reason
\\          three.o
# the next two objects must not be reordered
\\          four_a.o
\\          four_b.o
# end of reordering constraint
\\          five.o
#\\         six.o # removed until bug 1234 is fixed
\\          seven.o
\\          eight.o