not possible unless using an image crosshair.

probably more trouble than it's worth

basically you have to use a vtf crosshair
see:
http://teamfortress.tv/forum/thread/9842-rainbow-konr-wings-for-everyoneimage-to-crosshair/1
http://teamfortress.tv/forum/thread/12383-real-custom-crosshairs-from-ozfortress

if you have a hud crosshair you want to use, you can try installing the font so you can type it in photoshop, then save it as a vtf

make two vtfs, one for blue team and one for red
follow the instructions in the first link, except replace the code it says to use with this:

	"Crosshair"
	{
		"ControlName"	"CTFImagePanel"
		"fieldName"		"PlayerStatusClassImageBG"
		"xpos"			"c-64"
		"ypos"			"c-64"
		"zpos"			"3"
		"wide"			"128"
		"tall"			"128"
		"visible"		"1"
		"enabled"		"1"
		"scaleImage"	"1"
		"image"				"replay/thumbnails/[crosshair_filename]"	//shouldn't matter whether you put red or blue file here
		"teambg_1"			"replay/thumbnails/[crosshair_filename]"	//same as above
		"teambg_2"      "replay/thumbnails/[crosshair_filename_red]"
        "teambg_3"      "replay/thumbnails/[crosshair_filename_blue]"
	}

(insert the name of your vtf files)

Is there any way to change the transparency without going into photoshop and doing it manually?

don't think so. maybe if you open it in vtfedit there might be something there, but i'm pretty sure you can't through the hud files

couldn't you make a crosshair in your hudplayerhealth.res and make an animation that makes it always active?

Wouldn't putting "alpha" and then a following number between 0 and 255 change the transparency? I'm not too sure how that command works for images.

the command wont work for an image put in vgui/replay/thumbnails i've tried already

Thanks for the confirmation.