I don’t see any real workaround apart from recreating the whole recent files docker, which is definitely not ideal.
I believe a good way to do it would be to add a method to the Krita class to the python api, as this class already have the recentDocuments() method which returns the recent files list. The new method could either just append a new file to the list (something like appendRecent(str) ), or a maybe more modular approach would make the method override the current recent documents list so it would work in conjunction with the already available recentDocuments() method.
recentDocuments() is basically a convenient wrapper around readSetting("RecentFiles", f"File{x}", None) so it replaces all this code
from krita import *
krita = Krita.instance()
max_recent_files = krita.readSetting("RecentFiles", "maxRecentFileItems", "100")
for i in range(1, int(max_recent_files) +1):
file_name = f"File{i}"
setting = krita.readSetting("RecentFiles", file_name, None)
if setting == "":
break
print(file_name, setting)
This means the recent file list is just a setting as far as Krita is concerned and you can change it using writeSetting method letting you not only add more files but also change order or insert them in arbitrary positions if you wanted to.