Will it hold? Let me put it this way, following the BMW factory guidelines for slicing the front frame rail on a 2010 7 series you cut the frame rail off a little behind the rad support. You get the new frame rail and cut the part you need off it. They have a sleeve you stick into the frame rail. You bond the whole thing in there with structural adhesive with NOTHING but the adhesive holding the front of the rail with bumper, rad support and all mounted to it, is that strong enough?
Yes you could bond it all in. But I would think more about making something that your speakers bolt to in such a way that all you need is a few bolts here and there to hold it in. The bolts can even go where there are holes already. I have found that there is usually enough holes all over the car to do things like this, new ones don't need to be made if you are after keeping it all original.