The Comptroller's Office *had* the 502LU ready within a couple weeks of the governor's announcement. It had been pushed out to tax software providers for testing, and had passed. Then the feds put out their own version of unemployment exclusion, with a different exclusion amount and a different income threshold. Back to the drawing board.
We have a drawer full of returns with MD unemployment. For most of them we've filed the federal and put the state on hold. We call or e-mail each one every couple of weeks to keep them up to date. And since MD pushed the regular filing deadline out to July 15th, there's no need to file an extension. The biggest inconvenience for clients is that they have to sign the EL101 authorization to e-file separately from the federal 8879. They're just happy that we're watching out for them and keeping abreast of the many, many recent changes in tax law.
As to the suggestion for creating another subtraction on the SU, keep in mind that the SU relies on the preparer or individual to determine whether they're eligible for the subtraction. And the state didn't want to rely on people getting this (now more complex) calculation right.