Does it make it any warmer to wear a polyester rash guard under ones wetsuit?
Like edt said. If it is a polypro (fleece) lined rashguard, it will. Like wool, polypro retains a high majority of its insulating property even when wet. Also It is also hydrophobic and will push water away. An extra mm of polypro does not have the same insulation as an extra mm of neoprene, but it still is providing additional insulation without the added weight and restriction. This is why a lot of suits now are lined with polypro. An unlined rashguard will not do much since it does not have the same insulating properties as polypro.
On the outside it would do the opposite. Most wetsuits for wind sports are smooth skin unlike surfing suits. Surfing suits have nylon (nylon 2) usually on both the inside and outside. This provides durability but sacrifices warmth. In a windy environment the water retained by the nylon on the outside of the suit evaporates as the wind blows across. This takes energy cooling you down.