I really need something to bike in, but I'd like to use one of the lights I already own that are rail mounted.

Any recommendations?

IS there any way to position that light at an angle I like? (as opposed to a fixed rail?)

I don't think I've ever seen a cycling helmet that didn't have tons of ventilation holes - why not just run some zipties or paracord through the holes and fix the rail in a position you like?  Repurposing a head lamp might work also; most of them have a built-in angle adjustment.

I thought about zip ties, but I'm using a borrowed helmet, so I might as well buy my own that's more purpose driven to use in ... other situtations.

Well, you can either use a bike helmet in "other situations" where it is entirely inappropriate and could be the cause of serious injury.

Or.

You could use a helmet designed for "other situations" where it is entirely inappropriate on a bicycle and could be the cause of serious injury.

Right tool for the right job. A lot of science goes into keeping your head safe based on environmental risk factors. Screw with it if you want, but don't be surprised if you end up with undesirable results. There's a reason you don't wear a bicycle helmet on a motorcycle, and wouldn't wear a motorcycle helmet while rock climbing.

Your best bet is to get something like a Protec or PT Alpha. Both of which are designed for skateboarding and snowboarding where whacking your head on the concrete generally occurs in a different manner from that caused by a bicycle accident. They work fine as a general bump helmet.

I have an alpha PT I use for mountain biking in the winter. Flashlight on the rail segment and Inova 24/7 on the back with blinking  red lights.

What can I say, I like to bike and it is dark here at 1630 this time of year

I'll post pictures later.

good advice, I do appreciate that.

My question was prompted by a good price I found on the pro tec ace sxp, which is a helmet pretty well suited to my biking needs, but I'd need to attach a rail section, like maybe a L5 magpul or something like that.

How about the OpsCore FAST base jump helmet?  I know Giro makes bike specific helmets that cost more than it and I'm sure it would provide the same level of protection as those.

There are a lot of adhesive mounts that would stick to a bike helmet, or get a visor mount.  I use both for my Contour camera.

Protech Alpha and check out the Energizer Hardcase Tango, it has a helmet mount that rotates, locks every 15 degrees or so, instead of the fixed Surefire helmet light.