The nsIObliviousHttpService.newChannel interface can be used right away for making requests to arbitrary Relay/Gateway configurations, but it requires custom fetching and management for keyconfigs.
As we expand our support of OHTTP to more components it would be nice to have a common logic to manage configs:

Random list of requirements:

• Fetch keyconfig and store it in a memory cache
• Provide API to get cached keyconfig
• Provide API for clearing keyconfigs when clear-site signal is issued
• Get a signal from ObliviousHttpChannel when decryption fails because of a bad/expired keyconfig so we can refetch the updated keyconfig.

I wrote https://searchfox.org/mozilla-central/source/netwerk/protocol/http/OHTTPConfigManager.sys.mjs , which ticks some but not all these boxes. It may need to move to C++ and/or grow an idl interface if we want to use it from C++ consumers. For now we'll use it directly from the JS OHTTP shopping code to handle some of this.