As far as I know no membership needed. When you look at the map the route marked with black the Cataraqui Trail is open for all to use. Though it should be noted the map is screwy as it does not end at Newburg but further towards Napanee at Strathcona at the paper mill.A bit of a question mark for bikepacking - the Eastern Ontario Trails Alliance (www.thetrail.ca) bills these as multi use trails, but they are basically snowmobile/atv trails, some of which may be impassable in summer. A lot of them are rail trails. It's hard to find on their website, but a trail pass for a bicycle is $50 a year, or $5 per day, but I don't think there is much in the way of patrolling.
Their interactive map is at Maps
I may do some exploring of the Cloyne to Coe Hill trails this summer, but I read an account of a trip by trail motorcycle on some dirtbike forum that sounded like it is a tough slog at best.
Hopefully some of these trails could be developed for bikepacking excursions. If I get out that way, I'll report on trail conditions.
It should be mentioned that the K&P trail not shown has been pushed further north past Harrowsmith with the long ter, goal of getting to Sharbot Lake.