I've seen nothing on a trilled "L" sound anywhere. I've tried producing the trilled "L" sound and I can get something that seems similar. Is it possible to trill an "L" and if so are there any languages that use this sound?
It seems to me that lateral and trill are mutually exclusive - trills require repeated opening and fully closing, while laterals require a state of half-closure. If you drop the sides of your tongue to make a lateral, you make trilling impossible.
There are lateral flaps, though, which seem to be not uncommon - Japanese has one, /ɺ/.
There is an Alveolar Lateral Trill, it has this symbol; /rˡ/ and there a video of it; http://www.youtube.com/watch?v=kr4_zYBRaWY&api_format=3&vndel=watch&app=desktop
I can do something I'd categorize as trilled L: I close the air flow in my mouth by placing my tongue behind my upper teeth; then I blow the air out, creating very small opening on the sides. It sounds like a child mimicking a fart :/ It can be either a trill or fricative, depending on how the air gets out. So, I think it's possible. :-) I've never seen it in any language.