Hi Gareth - I often plant little things instead of just putting flowers down but I saw that others had also done so , so I assumed it was ok. If there are other examples nearby I expect it isn't an issue but as everyone else has said, it's probably best to check first. Sad to think that anyone could ever object isn't it?
I put a little viola in recently and although the ground is quite dry because of overhanging trees it'll survive longer than cut flowers. I put in a little Saxifrage in spring as it copes there too. There's bare soil between the graves and the grass so they're quite safe. Snowdrops are a lovely idea Welshonion.