Hi Steve,
The string you're looking for is "?asset_ei_screen=screenname" (ie. the name of the screen in lower-case), which is the standard usage for accessing a screen in Simple Edit mode.
Note, however that using this for preview mode only works in EES 1.5.0 [which requires a Matrix upgrade], and there is a small issue I can see when using this for the Contents and URLs screens (as their backend equivalents are called something different or provided through another asset), which I'll report shortly. The other screens are fine.
The hash method won't work, because the Login Design does not pass through hashes when it redirects after a login - only query strings. The hashes should work correctly once logged in.
Hope this helps a little.
Regards,
Luke