I've been landscaping my back yard wearing shorts most of the time and my shins have the battle scars to prove it, so I feel your pain!
Ex Officio makes several model pants with UL fabrics. They also offer some with permethrin treatment to battle the bugs.
Other than RailRiders, Solumbra makes (expensive) vented pants: http://www.sunprecautions.com/product/21300
Columbia Silver Ridge pants have a fabric that works well for me with a very dry feel and durability has been good. Their zip-off models have no ankle zippers, so one less thing to kneel on. Giving your pant leg a tug before kneeling is one way to get more slack and clear the zipper from harm's way.
UL tall gaiters are hard to find. I think the trend is to short gaiters for UL use and more armor plated versions for with standing ski edges, snowshoes and crampons. All the coated models will be hot, so that would be one step forward and two back. It sounds like a MYOG project otherwise. Old school gaiters were often made with 60/40 cloth, but they would still be hot. Perhaps Dirty Girl could custom make a tall pair for you?
I liked the tall socks idea for skin protection with some prayer of breathability. Cheap and easy too. The Boy Scouts, British troops and kilt wearers have done it for decades if not centuries.
Over the years, a few manufacturers have made pants with buttons and loops so you could roll up your pant legs with some hope of having them stay up. It wouldn't be hard to add the same to an existing pair.
Lots of durable rain pants, but not light ones. GoLite Trinity pants are 3 layer with heavier fabric in the seat, knees and ankles and are 13.4oz in a medium size (and discontinued). I've been of a mind that cheaper rain pants like Red Ledge and Sierra Designs are basically sacrificial stuff-- use 'em, patch 'em and toss 'em. I find Kirkland brand (Costco) rain pants fairly often in thrift stores and they make a cheap sacrificial rain pant option.
Once you get to rain pants, the lighter breathable gaiters are a more practical option for lower leg protection--- assuming it is colder when wetter. Outdoor Research has lots of options: http://www.outdoorresearch.com/en/or-gear/gaiters.html