Jeremy,
Only you can decide if it is going to be OK. Just remeber BE SAFE!!!! That is 350 degree hot plastic that you are putting under pressure. If that seringe fails and that plastic hits skin, its not like it burns for a second and stops. It will burn and burn and burn. You will be sorry. So you decide. Anything can be done safely. Even if yours fail you can use measures to protect yourself so its all in your hands. I believe Lurecraft or JansNetcraft sells a plastic injector for just over $10.00 that is made to stand up to plastic temp so if I was doing it I would go that way. In fact I have two of these that I purchased with intentions of doing just what you want to do. Then common sense got the better of me and I found the posts on here talking about vacuum pouring, and that just sounded like a way better deal for me and my 10 year old son. I couldnt live with myself had one of them failed on him, and he enjoys doing this with me way to much to not include him so we just needed a safer way. Good luck and post your results.
MT