I don't have one anymore to test though, but I used to use the mac layout on Linux for a while exactly for composing using alt.
Or maybe this has changed (back) in a recent OS update? I'm running 12.0beta on this machine.
The dead-keys like Opt-E for acute only work with a small set of letters in the standard US keyboard layout, although it's possible for a different layout to support more -- subject to the combinations existing as precomposed characters in Unicode. (For other combinations, you'd need to enter a combining accent after the letter, and rely on the font to supports placing it properly.)
edit: Looks like it's supposed to work, I don't know what I'm doing wrong: https://help.ubuntu.com/community/GtkComposeTable
edit: hold on, that's only in Firefox. Anywhere else, comma+c does ç and apostrophe+c does ć
If you’re not in en_US you can find other locales and their Compose sequences in the nls directory.