Mellotronists

One of the first things I did after buying the Mellotron
was hook it up through the Roland Space Echo.  I thought
that might be a little too much tape happening but it sounded
great.  The Mellotron is so analog you can probably get away
with judiciously chosen digital effects but many of the vintage
devotees probably shy away from that to the greatest extent 
possible.  
I do think a delay effect is beneficial, beyond just basic
reverb.  
Right now on ebay there are three of the MXR green analog delays
that I believe were based on BBD technology with some use of
now-obsolete ICs.  Pretty popular but no personal experience to
vouch for them with.