Does a mechanism currently exist for doing a sharedrop on UIA holders?
Nothing yet. At least not in the nice intuitive way where you just specify the amount and type of the dividend asset and the type of your UIA that you wish to distribute the dividend to (also nothing where the dividend issuing transaction size is fixed, and thus the transaction fee to issue the dividend is fixed).
You could always run a script that takes a map between addresses and amounts and makes the transfers one-by-one to each of the addresses for the corresponding amount of the UIA.
Then there is
wallet_asset_issue_to_addresses. But like the description says wallet_asset_issue_to_addresses doesn't "do any magic for you" it just helps do the work of the script I discussed in the previous sentence on your behalf.
Of course, there is the possibility that the transaction exceeds the maximum transaction size (if there are many addresses to pay), in which case you would have to break up the map into pieces and use wallet_asset_issue_to_addresses multiple times (and pay multiple transaction fees).
Finally, you need to generate the map in the first place. It would nice if there was blockchain_generate_snapshot command that worked for any asset rather than just BTS. I'd imagine that it shouldn't be too difficult to create such a command. It seems like one could just copy the chain_database::generate_snapshot function and make some tweaks to support any non-BTS asset (it wouldn't need to worry about delegate pay balances, vesting balances, short order balances, or collateral balances, but it should be careful to pay attention to both asks and bids to not miss any markets that are trading the asset). One thing to wonder about is what would happen to assets protected by multisig or stored in escrow? I guess worst case scenario you tell your UIA holders to not hold the UIA in any multisig or escrow at the time of the pre-announced snapshot. Although it would be better if wallet_asset_issue_to_addresses allowed for reconstructing multisig withdraw conditions (then the snapshot generation tool would be augmented to support recording multisig balances and escrows could be treated as a 2-of-3 multisig).
It would be the best way to handle this but I don't know how we can do it. If its not possible to do at the moment then our options are to either auction off the IOU's and distribute the proceeds when we re-enable trading, or to take a snapshot and then do the sharedrop to the snapshot addresses at a later date when sharedrop tools have been developed.
Actually, yeah, I think it makes the most sense to just announce a snapshot and then resume the markets at a new NAV of 0.7 so people can withdraw their portion of their funds as soon as possible. The sharedrop would happen later (when there is a better support for it) but presumably the BTC repayment on the IOUs would take a while anyway.