Pickup alerts would definitely make a difference - even if I don't see other players during the game, it'd be good to have a feel for where they were in the city. If there's a Twitter update of "Player Gwyn just made a collection from Marylebone Mag Crt", then that tells me a lot about where you are now, where you might be going next, and (if there's a scarcity of parcels) whether I can get to a different pickup point before you.
I'm not sure I see how "each job available only to the first person who picks it up" would lead to clustering, in itself. A clearer way to get players into the same kind of area might just be to weight the randomiser to always select a pickup point within 500m of the previous dropoff.
Games moderated by a Twitter server sound like a promising direction of exploration, particularly the idea of tracking imaginary props. Are there any Android/iPhone clients that can be geared up to audibly alert the user when a particular Twitter user posts a message?
It'd be possible to split the difference with "more points for the first arrival", but that seems to break the theme a bit.
You'll have to pull over to read messages, and you won't know whether your phone beep is a new job (worth stopping to recalculate and maybe divert), or just a distant pickup notification (possibly just noise). This could become annoying.
Come to think of it, I think this would be better as a team game. I'd like to see proper team identities, costumes, etc (since it's a long game already, this could make a day out of it). Stuff like:
DARKWATER INC Slogan: "We'll take it from here." Gear: Quasi-military Transport: Mostly runners and high-end road bikes. Music: Heavy metal. Respect generated through: Unquestioningly executing orders. Style: Command-and-control. Uses a central dispatcher, extremely terse CB language, and a squad of succeed-at-all-costs couriers.
DELIVERATORS UNLIMITED Slogan: "We're already there." Gear: Post-apocalyptic anarchist. Transport: Exclusively beater bikes and BMX. Music: Hiphop. Respect generated through: Speed. Style: Highly autonomous individuals with a mesh network. Extremely fast, Deliverators focus on quantity rather than high-scoring jobs. The company sometimes overcommits.
MACHIAVELLI LOGISTICS LLC Slogan: "One step ahead." Gear: Suits. Transport: Tube, bus, taxi, foot. Music: Classical, if any at all. Respect generated through: In-depth knowledge and strategic planning. Style: Large network of well-connected individuals. Slow-moving but highly organised. The standard against which other companies are measured.