Is there a particular reason you think this net's fanout is too high? Have you looked at the timing report and found the routing delay is too large? If it's not, then I wouldn't necessarily worry about it. The more constraints you apply to a design, the longer the implementation runtime.
However, if you are having trouble meeting timing and want to make an attempt to guide MAP to not prune your signal, try using a FROM/TO/THRU constraint in your ucf. "keep" or "preserve" attributes you use in your code to direct the tools are only used by the synthesizer. The synthesizer will not forward annotate those to MAP.