Just wondered how you could splice onto an Overloaded spell, and whether that would be any good.

Splicing Goryo's Vengeance onto an Overloaded thing would be funny :) Or, more amusingly, Psychic Puppetry. Doesn't quite work with Shifting Borders.

I don't think this is a good idea. Adding all the existing splice-onto-Arcane cards to absolutely any instant or sorcery is a recipe for big trouble.

As discussed over on Clockwork Souls, cards with "splice onto sorcery" could be carefully worded to work with overload. Alternatively you could just rule that text spliced onto an overloaded spell isn't affected by the overload replacement.

This may be a good time to mention one of my favourite decks from ages past, by the way, which revolved around using Splice with Ink-Treader Nephilim. I cast Kodama's Reach, splicing on some subset of Spiritual Visit, Vital Surge, Evermind... and Glacial Ray targeting the Nephilim. This draws me N cards, gains me N tokens and 3*N life, puts N lands onto the battlefield and N more in my hand, and kills all the X/2 creatures on the board :) It was a delightful blast to play.