The Icom 756Pro reflector had some info on this a while ago. As many users have noticed the power out on 5Mhz was only about 50W even though the input power was the same as for a 100W out.
As the Icom message indicates it was being wasted by the output filters. About 4.7 Mhz seems to be the upper limit and then it picks up again in the low 6Mhz.
Since the regional SHARES freq is in that area too I just use my Harris, Sunair or Raytheon radios that are designed to cover all freqs. Some of these rigs can be purchased for very reasonable costs, or via the MARS equipment program.
For example the Raytheon Ray-152 was being sold in quantity from a local surplus dealer for about $200. It was in very good cosmetic condition but needed new PA transistors and a new keypad. The transistors were $35 for the pair and a new keypad (from JRC as they are who really makes the radio) was less than $20. After that I had a nice 150W all mode, 1.6-30Mhz rig with a standard .3PPM TCXO.
I figured it's cheaper than abusing my 756Pro and 746Pro.