Starting this in an attempt divert the hijack of the Casual Bombshell thread.
Here's how I see it. Note that my statements, especially the ones about folders, are presented as observations and opinions not as fact.
The original idea of the Mules was to allow folks to experience, and "steel head" to compare, a variety of steels in otherwise identical blades. Some would be steels Spyderco has not used, some would be steels which they have. The hope was to make them as inexpensive as possible and, for that reason a fixed blade without scales was the chosen platform.
Making a "folding Mule would involve some tradeoffs and constrants.
The model would have to be one made in Golden, if only for logistical reasons.
The knife would probably have to be fully assembled or, at the very least, a kit where all parts were supplied. Either would, as a general rule, add to the cost.
There may be legal/liability reasons why Spyderco would not want to sell it as a kit.
However, if a kit were the choice, the FRN UKPK would be the most logical platform due its combination of price point, screw construction, minimal parts count, and (relative) ease of assembly.
Most other Golden models are either pinned, or considerably more complex. Remember, nobody ever loses a part when assembling a kt, the manufacturer left it out.
The possible up side would be that, when the most expensive exotic steels were used, an FRN UKPKr might cost less than a FB Mule. Good damascus, for example, can cost $15 or more per inch.
Ok, those are my thoughts, time for yours.