Somebody at an isp could knock up a simple script to log all accesses to Pprune
That is completely and utterly incorrect. Somebody at YOUR ISP could log YOUR accesses to PPruNe, whilst YOU are using YOUR ISP.
The suggestion that you're "home and dry" if you had the PPRuNe access logs is nonsense too. The PPRuNe log file has hundreds and hundreds of lines per second - marrying those up to a particular post would be a lot of work and doing so conclusively would require some luck (that nobody else posted at the same time). Even then you'd only have an IP address. Empirically, 95% or more of PPRuNe's visitors don't have their own IP address, so you'd then need to find out from the ISP who was logged on and using that address at that moment. They won't tell you without good reason, of which this is not one, so you'd have to hack them too. Seems a bit far fetched.
If someone posted something nasty enough about their employer to warrant all this don't you think that some amount of disquiet would already be apparent in their demeanor?!!! And if not then what difference does it make?