Adding a maintenance page to the pool

There must be other factors involved. The [HTTP::host] evaluation didn't break anything, simply because it didn't return the information the evaluation cared about. The active_members command looks at the count of active members in the pool provided. You provided it the host header of the request (which ironically should have broken it for semantics reasons), which simply didn't return a value that was less than 1. If the [LB::server pool] command is now breaking it, try supplying a specific pool name:

if { [active_members my-pool] < 1 } {

If that works, then you're best bet is probably either a) no pool assigned to the VIP, or 2) some other conflicting information in either of the iRules.