I am not familiar with Java application servers, so please excuse my ignorance.
Is it possible to schedule the garbage collection? If so, you could temporarily disable the server, kick off GC, then re-enable the server. HAProxy has a stats socket that would allow you to adjust the server's weight to 0 temporarily. If you could make a JSP to kick off GC, then you could have a simple cron job that uses socat to disable the server, curl to hit that page, then socat to re-enable the server. Do each server in turn (or on separate intervals). If you can do this more often than it would happen "naturally" then you can control the process and lose 0 requests.
