I quite agree that doing away with class skill restrictions would take the wind out of the sails of Multiclass feats.
For Warlords you could use the Military skill, which is my house-ruled knowledge skill to cover martial knowledge and scouting, as well as monster lore for natural humanoidsYou would then only have to designate the default skill for fighter, warlock, and warlord. Endurance for the martial classes and arcana for the arcane one seem appropriate.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.