Sorry if this has already been asked.
Who is responsible for fast pool switching? The proxy or the rig?
If I rent a rig running a custom cgminer/sgminer/bfgminer/whatever that has been configured/patched for - say - 30 seconds pool switch. Will this rig switch pools faster than a rig running standard cgminer 3.7.2 with 300 seconds pool switch?
Yes, it will be based on the settings in your miner. I use Kalroths cgminer v3.7.3 and i think he set the default there as check every 60 seconds, worked flawlessly this morning for my first customer.
Please correct me if I'm wrong, but to my understanding the rig has a permanent stratum proxy connection and does not know where it's mining and when it's switching pool, so why should a custom cgminer switch faster than a stock one?
Sorry, by "if i rent a rig" you mean rent from, or put available for rent? If you're enquiring about renting for your own use, then i'm sorry, i misunderstood.
In this instance, i guess it will depend on how fast the server on BetaRigs updates its proxy software to use the new address, because on the rig side the betarigs address is static. So, i would imagine it's pretty fast. This is just a semi-educated guess though, since i have no idea of how BR have things configured.
But, using a custom cgminer will allow them to switch faster from their own mining pool, to the BetaRigs pool. Which means once your rental period starts, their rig will hop over to mine for you sooner. So at the very worst it could save you some minutes of otherwise lost mining allowance.

Yeah, that's exactly what I'm thinking, only the first switch is faster with custom cgminer. Time required for every further switch is driven by the proxy. Maybe mux can shed some light on this.