I am mostly using locked folder tabs so I'd like to be able to replace the icons with unobtrusive alternatives.
It even seems like it was possible at some point but it doesn't work anymore.
Otherwise, I think an option to hide the icons altogether would also be fine.
I really hope, very much, that this is a simple enough request to be implemented in an upcoming beta.
And if it's not, or probably not, let me know please. 
The "it was possible" thread was about something different. The lock icons have always been shown (for locked tabs) but there was a time when the icons for the folders themselves didn't use the folders' real icons and were always generic, which I don't think is what you want.
The "it doesn't work anymore" thread is just some toolbar icons and an example menu and button which use them to lock and unlock tabs. It has no effect on the icons shown on the actual tabs.
There isn't a way to completely hide the tab lock icons, but if you turn on Preferences / Folder Tabs / Options / Small close buttons then they'll be less obtrusive, since they become translucent and share the space with the close button.
Okay, so there's an option to get rid of them 50%... We're halfway there! The other half could be dealt with by an advanced setting like "tab_icon_transparency" : "100", would that work?