Lavatory kit
A small kit to take care of any (either) problem is easy to put together.
Standard (TSA approved!) 1 qt ziplocs, diaper gel (safesorb or other binding powder, safesorb.com, a hospital/lab product), two large garbage bags, a roll of paper towels, some baby wipes, a pair of rubber gloves, and some tie wraps.
I have a small cooler (12 can) that doubles as a box for ropes, oil, funnel, etc, but it can be emptied.
To micturate enroute, put the safesorb in a plastic ziploc AFTER testing it with air for leaks, and place a towel or two in the lap for a barrier. The gel absorbs the urine quickly, giving a hard snow consistency. Place it in a separate ziploc for safety and stow in an opaque bag for disposal later. This can be done by the motivated female, I promise.
If the undesirable is inevitable, line the cooler or other bag/box with the garbage bag, and offset front seats so you can hang a cheek and place yourself over the open bag. Some safesorb (or towels) will help if consistency is, er, thin. The gloves help stay clean and the baby wipes are helpful later (toothpaste works as hand soap in a pinch). The bag can be closed with the tiewrap and then placed in a second bag with a closure, making the rest of the trip bearable.
Except for the "box" the whole kit fits in a single ziploc.