I have very little experience with this, but I have done the following twice.
After I sparge my mash and lauter out what I need, I let the mash tun sit with the little liquid is left for another 30 min or so. I don't get much more out of it but I am usually too busy with my boil to mess with it right away. I then drain the mash/lauter tun into a sanitized gallon jar, and purge with a little c02, and cap. Since the ambient temp of my garage is usually between 30° and 38° right now, I leave it in the garage next to the AG system. I also brew every two weeks, so about Thursday before my next brew on Sunday I bring in the wort to boil it for a few minutes, cool and pitch my yeast vial into my flask with the wort.
The last brew I did I had and OG of 1.043 go to a FG of 1.009 with a starter made this way. The starter wort had a GU of about 25, and the yeast was bubbling normally after about 18 hours on the stirplate.
Just a different experience. I haven't tasted the finished beer that used this method of making a starter, but the first is just about ready to force carb. Tasting the samples I do for gravity readings doesn't lead me to believe it is sewer material. In fact it actually tasted pretty good the last sample I tasted. I am going on the fact that a starter is primarily to wake up the yeasties and populate them--any amount of fermentable sugar will accomplish this to one extent or another.
Now with regards to did I have enough of the little guys with regards to over/under pitching I am still working out using the tools to do so.
But, like I said, I am just experimenting due to the necessity for having wort for starters and not wanting to buy LME like the OP desires.
Will I blow anything up by doing this?
Nah.