Cinch 2.4.0 adds support for spoofing nicknames using the draft/relaymsg capability and its RELAYMSG command, introduced in ircv3/ircv3-specifications#417.
To do so, pass a nickname to Target#send, Target#safe_send, Target#action or Target#safe_action using the new optional as keyword argument. The IRCd must advertise support for the draft/relaymsg capability during capability negotiation, and, depending on its implementation and configuration, may require the bot to be an IRC operator and/or include a nickname separator character. Ergo is one IRCd that supports the current draft spec.
Target('#foo').safe_send('Hello world using RELAYMSG!', as: 'FooBot/Bar')
#=> RELAYMSG #foo FooBot/Bar :Hello world using RELAYMSG!