I did get my phone unlocked, but my situation is a little different than yours.
I was on T-Mobile's prepaid for almost a year, but switched to a Simple Choice plan on April 1st. So I ended up needing to wait forty days under the post paid plan before I could unlock my phone. I ended up unlocking not my 810 but my older HTC HD 7.
So I was on a different plan, but it really just ends up taking twenty days off of the "required" time that it would take on a prepaid plan.. I did not have the same time constraint you did with your trip to Europe.
I am assuming that you have not yet been sucessful in getting the unlock for your phone. It is too bad that you could not get someone in T-Mobile to advocate for you and argue your case. I prefer the type of customer service wherein the customer service representative "takes ownership" of your concern and coordinates your issue to resolution.
Unfortunately I find that a large percentage of companies have less than stellar customer service. The idea that T-Mobile's own documentation states "or for Prepaid service, have used your device within the past 60 days" points out one or several different problems. First, perhaps the document is written poorly and it really does need to say "have used your device FOR the past 60 days." Or if the document is correct, then the issue is an failure of communicating policy within T-Mobile's different departments.
Neither alternative is particularly impressive.
I do wish you luck, however I would assume that you are in Europe and the pressing need has in a sense passed. I do hope that you enjoy the 810. I owned one and I liked the phone. However I found the 8 gb of memory to be insufficient and I have questions about what type of firmware support it will get now that it is EOL.