What I'm doing - and it doesn't work for ALL secure lits - is using ConvertLit to downconvert the secure lit to an unsecure lit, and then using Book Designer to convert the unsecure lit into a libre lrf.
The results are pretty good. You'll want to play with the output format a bit since the libre fonts are a little different from the reader fronts and so you can get what the software thinks should be a single line wrapping across into a second line.
I prefer my fonts to be a bit larger so the output option I chose are:
Book style: Simple
Book Text: 22
Book Cover: 32
Paragraph Indent: 5
Margins Left: 5
Margins Top: 0
Page Width: 600
Lines per page: 20
And under the Option tab, I deselect Hyphenate.
What it produces is quite acceptable, but, of course, not 100%. It drops any accented characters, and doesn't convert open and closing quotation marks as two types but just makes them both the same - " - but it does provide a nice readable output.