Which skill combos of which professions can reliably achieve an effectively permanent swiftness boon through slot and weapon skills, without chance skills or traits? If there is none, which combos have the highest percentual swiftness duration per cycle?
Elementalists have several ways to maintain 100% Swiftness uptime.
One is to put 20 trait points into Air Magic and select the Quick Glyphs and Inscription traits. This will make the Swiftness granted by your Glyph of Elemental Harmony last as long as the skill's recast time.
Staff Elementalists can combine Glyph of Elemental Harmony, Windborne Speed, and Static Field+Arcane Wave (Lightning/Blast combo). This works regardless of what traits you pick, but you do need to carry Arcane Wave for the blast finisher.
Dagger/Dagger Elementalists can combine Glyph of Elemental Harmony with the Zephyr's Boon trait in Air Magic and their Frost Aura and Shocking Aura skills, as well as Ride the Lightning and Updraft. As an added bonus, 5 points in Air Magic lets you move 10% faster when attuned to Air.
The Speedy Kits trait grants Swiftness when changing kits.
All you have to do, besides having the trait, is keep on changing kits in order to keep your Swiftness up. You will need at least 1 kit skill in a utility slot in order to actually equip it. You can equip/unequip/equip the same kit to keep the swiftness flowing.
Staff Guardians can use Symbol of Swiftness, which grants 8 seconds of Swiftness with a 15 second cooldown. For utilities, you can use Retreat (20 seconds swiftness) and Save Yourselves (10 seconds swiftness). Using a single cycle of those skills together will provide a total of 38 seconds of Swiftness.
To expand on Engineers (PvP specifically), I've been running a PvP build that focuses on maintaining swiftness and have enjoyed it:
In a PvP setting, I prefer to not be caught off guard while switching kits, but for perma swiftness agnostic of combat effectiveness it is a viable option.