Frankly if the landlord is more concerned about the superficial appearance of the stairs rather than the safety of the occupants, there is not much that can be done. I'd inform the landlord that the stairs are slippery and unsafe and ask him to make them safer - in writing.

I found some anti-skid safety strips and tape but they are in a bright yellow - as recommended by ADA for visually impaired people for safety.- red or black. Grainger Industrial Supply

Lowe's carries an anti-skid additive that can be put into paint to make it anti-skid, but I don't think it is colorless. See below.

OOOHHH! I found this! "Seal Krete Clear Grip is micronized polymeric light profile aggregate for use as an anti-skid additive in most paint or clear coatings. It is hard, clear, and has excellent resistance to chemicals and detergents and will give gentle texture to your floor paints. It is effective and yet is gentle enough for walking on with bare feet. Excellent for around pools or food preparation areas or where often cleaning is necessary. The lightweight allows it to remain in suspension longer than traditional skid resistant additives. It is resistant to solvents and can be used in solvent-based alkyd, urethane and epoxy coatings. Do not use in repellents, silicones or wax coatings."

Here is a link that might be useful: Interior/Exterior Anti-Skid Additive Paint

Maybe call a carpet installer, explain and get an estimate.

'non slip stair treads'
I just did an ebay quick search...
16 new navy blue Non Slip Stair Treads.
These mats add a warm look while protecting wood from scratches, dirt and wear. Put in place for temporary use, or install with carpet nails or glue. Durable polyester with non-slip rubber backing.

9" x 28 1/2". Current bid $19.99
maybe something like that, or get some rubber backed carpet and make your own.

Here is a link that might be useful: Non Slip Stair Treads

There have been some really good postings in the several sites we put out question. Looks like the best thing is a clear spray with small particles that turn the wooden stairs into something that is less slippery since the landlord would not let us put carpet on the individual treads that make up the circular staircase. Thanks for your help!