Bards naturally have +1 (untyped) to all untrained skill checks (Skill Versatility), and a feat from Dragon 383 called Bard of All Trades gave a further +3 feat bonus to untrained skill checks. Note, however, that even without being a Bard, you can pick up Jack of All Trades, a PHB feat that gives +2 to all untrained skill checks; coupled with any other source of bonus (racial, item, consumable, etc.) this is usually enough to get by, so long as the check doesn't require training. For example, the Exceptional Factotum Helm from Mordenkainen's Magnificent Emporium gave a +1 item bonus to untrained skill checks (+2 as a paragon item, +3 as an epic item), and there were a couple powers that granted power bonuses to untrained checks. Getting some consumables that apply to specific checks would also make a big difference.
I don't know of any features which let you count a skill as if it were trained without actually becoming trained (even if temporarily), but if you could, that'd be amazing with a Bard of All Trades + Exceptional Factotum Helm + a Runepriest buddy (to give you Mark of Skilled Effort.) You'd be getting 4+(1 to 3)+5 = +10 to +12 on untrained skills once per encounter, which is perfect for ritual casting, while still being able to act as though you are trained. Even without the Runepriest friend, you'd still be effectively trained in every skill just with the feat and the item--focus your actual skill trainings on skills that need it, like Arcana and Religion, and let your raw talent take care of the rest.
Kind of reminds me of how there were ways to substitute Arcana for like half the skills in the game, and then to juice your Arcana into the stratosphere. A guy in my favorite (and sadly untimely-ended) 4e game was probably moving gently in that direction with his Wizard.