I agree that it is a difficulty having to haul mud for long distances and how retention of mason bees would have it's negative impact on local populations. I have retainer wall of cement blocks which raise the earth of border shrubs. Since my blocks are not cemented I suppose they find proper access to the right kind of thing in all the adjoining natural tunnels that are not occupied by our spiders.

Another hardship for all bees during rainy days is that the pollens get washed off the flowering fruit trees. There must possibly be recommended options of differing flowering types of shrubs etc to solve this kind of shortage during wet periods. 

Before the rains of the last couple days I pruned large branches from my later flowering plums and purposely placed those near my mason bee homes which are also sheltered together near the homes front door. Honeybees, any bees are not flying today, we even had hail. But in honeybees they always have spare stashes of pollen stored in honycomb for days like today to keep up with the non-ceasing egg laying activity of the queen. Mason bees are hindered in that area. I have had both. I will wait and see what happens next.

I put out a water heater drip pan with a mound of soil in it. It collects rainwater and the soil stays wet. Bees can get dry soil at the top of the mound or wetter mud down low. If they land in water they are able to swim and climb to dry ground.

Another thing I do is dig a hole under the eave of my chicken shelter roof, rainwater keeps the bottom of the hole wet and drier dirt towards the top.

Like davem, I too am "blessed" with plenty of clay.  This northern part of our property is mapped as "silty clay loam."  However, when the property was developed, all the "good" topsoil was pushed off to the side of the yard by Bulldozer Jockeys who know a lot about moving dirt, but don't have a clue about soils.  Much of the yard is scraped down to clay that I'm sure would make some nice pots, once they're put in the kiln....

Because I like to cultivate various species of plants that are "aquatic" or "emergent aquatics," I've purchased some cheap plastic "Kiddie Pools" to set plant pots into, or to partially fill with native soil.  If I use some soil, I put it in somewhat "high" toward one edge, and low toward the other.  That way, there is a dryish "shore side" and a saturated, inundated "pond side".  Of course, as noted above, one needs to make sure that a certain amount of water remains so that some of the soil is wet or at least moist.  

If there's a decent amount of clay present in the wet soil that is available, bees and wasps who like to use clay architecturally will find it a bonanza, and will visit it repeatedly for supplies.  Quite some years ago now, I was very pleased to find a Potter Wasp repeatedly visiting one of these "Kiddie Pools" that I was sitting next to at the edge of the yard.  The quality of the clay ball that it could gather was to it's satisfaction and it made repeated trips.  

On the other hand, one should be sufficiently active and observant around the yard so that when a bee or wasp is found floating in the water, you can lift it out on your finger-tip and let it dry-out its wings and recuperate until it can fly off... whether or not they give a "Thank you."  I find most of the rescues to be courteously thankful.  

Reinettes

I set up drip irrigation.  Dig a hole in a clay muddy area and use a hose timer to make sure it stays wet.  Bees actually burrow into the mud to collect it.  Dozens of bees at a time. I think, perhaps, that as with birds, a water source is a better attractor than food.