Am thinking about running a Magicshine front headbeam as well as a 2nd low-powered blinker aimed 45 to the right for side-approaching cars.

Good idea? Bad idea?

Any ways to even mount it?

A small led mounted on your helmet would serve better as a being seen light..

If you want to mount something at a 45 degree angle,it would seems mounting to the headtube would work best..

If you could get your hands on an old reflector mount, then you could attach the collar intended for the seatpost to your handlebar and use the joint intended for vertical angle adjustment to direct the light horizontally.

I grabbed a few of these from the LBS for free last time I was in, maybe you could have similar luck.

I use the MS on the bars and a Terralux on my helmet. The helmet light gives me more visibility to cars approaching from the side(s) and makes it easier to see into turns. Works great!

another nice option are adventure guardian lights, very small form factor and many ways to mount them.. Bright for there size and you can get a variety of colors..

https://www.botachtactical.com/novlabguar.html

Here's my really really ugly setup. But I guarantee you it works. And I've run it by several cops and they all give it the thumbs up as well.

This won't work for most of you, but if have a rear rack you can take a Dinottee light head and strap it to a piece of hose or PVC tubing and point it at a reflector, but facing forward. Set the light to strobe and you are now "noticed from all directions.
The light bounces off the reflector, and since it is behind you it doesn't blind oncoming motorists. There is so much "over-flash and spill it is impossible for people not to see you from all but "head on" - and probably that too! (this is a flash photo - the light would have been brighter) Tail lights just above reflector, under seat