So what's the reason for the dynamic mapping? Do you only have a single end point? Since you can apply a map(s) at a send port level, in a basic scenario, you do dynamic routing and whichever send point has the necessary subscription (e.g. Quantity > 100 AND VendorID == 1234) will be used and the appropriate maps applied. Also note that a send port can have multiple maps and the matching inbound root node will be used to pick which map to use.
To your other point, reading the file in repeatedly is indeed poor for performance, and, would require that file to be sitting on each machine in the BizTalk farm (unless you are using UNC paths). So, you could put those values in the btsntsvc.exe.config file (which is read upon host start up), but then you still have to keep those config files in sync on each machine.
The best choice for reading those types of constantly used (yet static) values is to use either the BRE, or SSO. For the SSO option check out the code samples online (http://msdn.microsoft.com/biztalk/downloads/samples/default.aspx), the one called SSO as Configuration Store.