This is the point of contention right there. If your original work contract doesn't include an agreement to drug tests, the employer would have a hard time sanctioning no-show, in many countries (when challenged in court). Constitutional laws do play a role, because sporting rules can be challenged in court by an athlete who feels unfairly treated. Just like work contracts can not specify arbitrary conditions (of course the legal framework for work contracts is much tighter than for sporting rules).
But looking at it this way, work laws have been used to challenge sporting customs/rules in the past, with the best known case being the Bosman ruling
(at least in EU). So I would not be surprised, seeing your proposed lifetime bans not hold up in court. Especially when it concerns doping cases (and lack of disclosure thereof), that are already beyond the statute of limitations by the sport's own rules.
Last post on this theme (feel free to PM me) as it seems only you and I are interested in this (perhaps the spectating public's "not another doping story its soooo boring" sets the tone for the teams, national assoc and UCIs obfuscation and denial?)
Couple of points: 1) The Bosman ruling was very different in its ethos - essentially that case was about about "human rights" (and, not in a small way, the European courts desperate need to justify their very existence). Very different from "how much can you punish someone for breaking their own contract..Great example is if a company director is negligent in managing his company such that it goes out of business, he can be barred from owning a business for 10 years (frankly a 10 years ban for a cyclist would be a lifetime
2) Even if Im wrong, I think "our ban might not be upheld by the European Courts" is a poor reason for not doing it, again, contrasts with UK athleticis which was quite prepared to fight in court its ruling of a lifetime ban on Dwaye Chambers