Since Plesk Onyx you will be able to install memcache as a Docker container and configure http(s) proxy rule for it to map to your web app.
Docker and proxy rules are available in Plesk Onyx Preview already, you can try it here https://talk.plesk.com/threads/plesk-onyx-preview-and-feedback.337172/
SEJan Löffler commented
sure Jan, with pleasure! We launched Plesk Onyx 17.8 last month which comes with free NGINX caching. You can control all cache settings easily by domain from the Plesk UI. Based on our intensive benchmarks with Varnish, NGINX, Memcached, Redis, WordPress Caching plugins (e.g. WP Super Cache, WP Rocket, W3 Total Cache) NGINX had the best out-of-the-box results. This is why we integrated it. With Varnish we achieved the very best results, but it requires a lot of experience in tuning. To simplify the very best caching, we just launched "Speed Kit" today which is Varnish and CDN based with an additional super efficient In-Browser Caching. This Pro Extension for Plesk was build with a Partner of Plesk that have put 30 man years in caching experience into the product. The result ist amazing. I would be very interested in your feedback!