Greetings!
Are you certain that this information is accurate? I have a fair amount of experience with fountain pens (typically vintage) and when I ask about special care instructions they mention tap water and warm water interchangeably. As Jockeys correctly said, you don't want to use hot water, but warm is quite acceptable to use. If you look at John Mottishaw's website you will find his instructions for cleaning a gunked/poorly functioning pen also includes soaking in a mixture of 1 part ammonia to 2 parts water. This is generally reserved for fairly gunked up pens, but can be used as part of a normal cleaning ritual. I personally prefer a weaker ammonia to water ratio.
There are of course certain makes and models of pens you cannot do this with, just as there are certain pens that cannot go into ultrasonic cleaners such as ones made from BCHR. However, from my experience (save extreme water temp) pen feed systems are pretty resillient. My primary desk fountain pen is a 1925 Waterman Ideal with a flex fine nib...writing with that puppy is pure heaven and I flush it with warm water every time I clean it (going on 4 years now).
As for the warranty being voided for using warm water, it piqued my curiosity enough that I sent an email off to Waterman customer service and will hopefully have a response shortly. I will post it here when they do respond.
I also strongly agree with Jockeys about heat causing the ink to spatter inside the cap. I have seen that happen many times to my own pens. It could also be that the nib is slightly loose, causing just enough gapping to allow an additional avenue for ink to expand/escape.
Warmest regards!
Charles