Personally, I think just allowing a player to "research" a new unique spell that exactly duplicated the effects of a metamagiced spell (including spell level) would be a reasonable solution. Making a sorcerer have the metamagic feat is kinda silly, I think, because they won't have the ability to decide which way to cast it. They MUST cast it at the higher level.
Now as for wizards doing this, I wouldn't allow them to do this with their free spells they recieve when they level up. Otherwise, I think it would be unbalancing.
But to allow a sorcerer to pick a unique spell that just so happened to function exactly like a regular 3rd level fireball spell, except that it was 7th level and had a casting time of "Free Action" seems reasonable to me. Your DM would be wise if he limited the sorcerer to only 1 spell that was a free action per round (just like quicken).
It seems entirely balanced to me to allow a sorcerer to create a new spell called "Quietman's Acid Arrow" that functioned exactly like Melf's version, but had no verbal components and was 3rd level.