It's blindingly simple. The trials have a stop date but no start date, so install and set the clock back a few years. Simple as that.
Even if they do expire you can set the clock back and reinstall them (so print out all the codes and keep them with the scope). I don't recall whether or not you need to set the clock to the validity window when initially installing.
For a long time I'd just assumed that they would have left some sort of marker to prevent re-installing once expired, and held off for years trying any until a time I really needed serial decode etc. for a job. Then for the hell of it I just tried one of the more obscure ones, waited for it to expire & found that it could be reinstalled if the clock was set back. So then filled it up with everything available - UART, SPI, I2C, Segmem, masktest, CAN, I2S & still working fine after over a year.
It's unlikely that there will be any more useful firmware upgrades for this line, so not a big deal never upgrading again & risking an upgrade that might stop this (e.g. like on the now MSOX, where you can't set the clock before the FW release date). I think I'm only about 2 updates behind the latest version, and those just added support for some extra autoprobe types. I don't know whether the latest FW has anything to make the trials stop - I think it was released after some discussion of this here so there's a very small possiblity.
What I found surprising is that when you request a trial, there is no license agreement/terms to agree to, and no folllow-up from sales.