Similar to editing lunar coins, it seems you can edit your userdata to give yourself Rebirth items. So far a pretty simple thing to achieve

All the steps are exactly the same as Lunar coin to start, so:

Navigate to your Steam library folder on the PC. By default, the path should look like this: C:\Program Files (x86)\Steam\userdata

Find the folder with the name 632360 and open it.

Go to the Remote folder, followed by the UserProfiles folder.

Locate the XML document there and open it with a text editor. I use notepad, but any text editor should work

Use Control-F and type "rebirth", it's the second one, but correct line should look like:

<name>Redcoat</name><RebirthItem> (Redcoat will be replaced with whatever your steam Username is)

next, edit the line so it now looks like this: <RebirthItem>ItemIndex.ResetChests</RebirthItem>

In this case, the chosen item should be the Sonorous Whispers.

Make sure to save the XML document first, then close it and the folder.

Start the Steam launcher again, and launch Risk of Rain 2 to check if the Rebirth artifact shows the correct starting item.

Through some quick testing, it seems that you can't just put the name of the item you want in between. It'll require more testing to determine, but maybe each item has a different name that usual? There's also a potential that it saved the ID elsewhere, and ItemIndex.ResetChests is merely referencing the last item you offered.

At the very least, this proves it's possible to edit the starting item. Just ya know, recognize that you're messing with your save and could corrupt it if you edit the wrong info. If that does happen, there is a 100% complete save that you can download from the steam guides, so there's no real risk to messing around.