LSD allows to specify the date of ancestral node via mrca(A,B,C) for the most recent common ancestor of A,B,C. To be honest, this is quite difficult to remember due to the mrca notation.

How about just simply use a list A,B,C?

Together with my suggestion about the rate range format, the example may look like:

5			# number of temporal constraints
A 1999.2		# the date of A is 1999.2
B 2000.1		# the date of B is 2000.1
C 1990.5:NA		# the date of C is at least 1990.5
D 1998.21:2000.5	# the date of D is between 1998.21 and 2000.5
A,B,C NA:2000.12	# the date of the most recent ancestor of A,B, and C is at most 2000.12

What do you think?