Doesn't seem to work like that Azriel, with IS it seems you are confined to the proxy utilized by your assigned IP.You cannot deliberately enter a new general public IS cache from my continued limited experimentation, there are a bunch in the 168 range that I have been funnelled through before. So it seems there are more extensive Access Control Lists and topology wize I know for sure that they SAIX proxies are transparent and utilize Netcache Netapp inline transparent proxy servers, whereas I have yet to establish whether or not the IS ones are transparent. RIght now, I would say they probably aren't because of the latency involved with transparent proxies, which they surely could not afford while utilizing satelite.
Not to mention I don't know all the IS proxies so my results are limited, unless they specifically say to you that there are other proxies for you to use when the network is slow in your vicinity.