"...compared to how much energy you get out of the fissionables useful lifetime. I would guess that a system like this would take a huge chunk out of the net efficiency of nuclear power."

That's what the high cost of storage of radioactive material does- lowers the net efficency of nuclear, or other words makes it less profitable to operate a nuclear reactor. I would be perfectly happy if the cost of storing nuclear waste was 10 cents a lb or less- that would mean it's completely unrealistic to ship to the Moon. But the point is how much is being paid now to store it- there is a considerable cost to just keeping this stuff on site- but important from a business point of view it's a liability and some degree an unknown risk in terms of cost.
The Lunar Disposal of Radioactive Waste company would provide a service which is they take this stuff at X price and thereby take the liability for it, therefore the cost of nuclear waste is knowable [just this fact alone is helpful in terms operating a nuclear reactor, because it lowers the investment risk of investing in nuclear reactors]. Now, there is a problem for the LDRW company- it has to be able to assume this liability- it needs to have the deep pockets so that the public/gov allows it to legally take this risk. This is such a huge problem that it is very unlikely that such a company could do this. But assuming you could get past this mostly political [not technical nor economic] hurdle, then it's a matter of what the price is. And as far as I know the current price would be at least couple hundred dollars per lb- it could $500 or $1000. But I'm pretty sure it's not $10,000+ per lb and so at the moment it's not close to being something that could be done.

But this a largely political problem- in other words it needs laws passed and these laws *could be* such that there is NO cost in short or long term to public [tax dollars or significant risk to health or environment].

"Besides, I hope we have some better uses for the moon in the next few centuries than dumping ground. "

Well, it wouldn't take this waste area at either pole, and I would suggest the site should be near an area in which in future nuclear fuel could mined from the Moon- and therefore close to where nuclear reactors might operate in the future.