I'm pretty sure this is already being done in doubles, giving Slaking a plethora of interesting abilities instead of its hampering Truant. However, their are a bunch of abilities you can't Doodle. Namely As One, Battle Bond, Comatose, Commander, Disguise, Gulp Missile, Ice Face, Multitype, Power Construct, Protosynthesis, Quark Drive, RKS System, Schooling, Shields Down, Stance Change, or Zen Mode will cause Doodle to fail if its the target, or an ally has it.
As for viability, it's really a mixed bag. Slaking without Truant is an okay Pokemon, but with the right ability it can be a busted one. But how often can you reliably get a good ability? Just having Grafaiai and Slaking on your team spotlights this combo as one of your goals. The Treasures of Ruin are also abundant in doubles, and their dark typing allows them to move ahead of Grafaiais Prankster and pretty cleanly ohko without a Focus Sash. Not to mention double Protecting turn one to wait out slakings Truant turn and block Doodle, to then switch into Mons with poor abilities to copy while Slaking loads would also wall this strategy well.
Not sure if the risks match the reward to do this viable but the strategy is doable and can lead to some straight up silly combonations.(Looking at You Huge Power Slaking).
Abilities you Cant Doodle
Hope I Helped
