The only modification to relays that I had to make was to have them verify and splice OTS proofs into the events they return. Here is a reference implementation:

https://gitlab.com/inkan_dev/ots-enabled-strfry

Inkan runs without operating a node, but I guess it has to trust some nodes along the way.

It's not clear to me why people would not want to opt into using blockchains if it works.