To check focus you could try the old trick of Scotch Magic tape put across the film opening in the back. Focus on the tape like a ground glass. Or use a small ground glass or similar plastic piece. If I recall correctly someone on Ebay is selling a Holga focus check ground "glass" for $10.
I've flipped the lens in a Brownie and have gotten interesting results. The Holga seems more fragile to me. I'll be curious to see how you did it what the results look like.