A smallish addon that started out with a third option in the LFM window (players/groups selection, added "all"), but then grew into sticky LFG/LFM while relogging, dinging and grouping with the option (via button on the LFG pane) of pausing your LFG/LFM choice and restoring it to the previous setting with one click.
It also tries to parse the comment of groups for "# heal"/"heal"/"# tank"/"tank"/"# dps"/"dps" and fills that into the columns. (And it parses a special comment "?D#T#H#-" into the columns, but doesn't offer a way yet to set that up via buttons...)
WARNING:
This addon does inevitably cause taint and blocked action messages due to Blizzard's (still faulty) dropdown menu implementation. There's nothing about it that I can do besides completely duplicating the whole LFG interface, and that's currently quite some more work than I'm willing to put into this.
It seems that you can avoid being blocked on all other unit frames by being blocked on one, therefore I suggest to focus *yourself* (with the menu of your player frame) to force the Blizzard code to initialize itself properly - then your target/party/raid frames should be working.
FLFTAQ (Frequently lazily failed-to-ask question):
"I just want the 'all' thing, can I avoid the other stuff?"
If you enable the storing of pause mode across sessions (with '/sanelfg config pausesave' or the cycle button menu), clear your LFM and your LFG choices, and then enable pause with an empty LFG/LFM sheet, then it won't do anything further besides the 'all' feature.
Future plan: Include a more comprehensive mixed mode where you select *multiple* dungeons and *all* are shown. (I.e.: Looking for trial of the crusader, normal or hero => show all groups for both.) This is not that simple due to some restrictions on the server side (hardware event requirement).