What would happen if I machine laundered dry clean only fabric before making it into a garment?
I think it really depends on the fabric.
If the manufacturer of the fabric states that it should be dry cleaned, that's probably a signal that they're serious about it. Silk and wool can often be hand-washed in very gentle shampoo as a way of pre-shrinking the fabric... but some fabrics simply don't tolerate washing well. They lose their look or shape when soaked in water, so even pre-washing won't do much for you. Another consideration is colorfastness... apparently some brightly-colored silks can lose their color when washed.
From a dry cleaner's website:

The first determination is whether or not the fabric can be submersed in water. Fabrics such as cashmere and wool are susceptible to losing shape in full submersion and will likely shrink, so typical washing should be avoided. Another gentle fabric is silk. An extremely delicate material, silk can be hand washed; however, it should never be complete submerged nor agitated in a washing machine.  [. . . ]
While some fabrics may be washed, like certain chiffon, satin, and lace articles, additional care must be taken when ironing. A dry cleaning process offers a safer alternative to typical ironing, making it more effective for a wider variety of products.

If you're making a structured jacket and you want to retain the crisp lines and smooth drape, you probably don't want to machine wash your jacket as it may destroy the cut and the work you put into it. Now, if it's a casual jacket that doesn't need to look &quot;neat&quot; or &quot;sharp&quot;, your washing regimen likely won't matter but keep that in mind.
I found another quote that sort of peeved me - it's tangentially related and I thought it might be valuable here...

Q: I was wondering, does it mean all the Big Designer Labels pre-launder all their fabric before production?
Graceful Sewing
June 1st, 2015
No, they do not. That is often the only reason that garments purchased from them have to be dry cleaned. If they had preshrunk the fabric, we could toss it in the washer and it would do fine. But because they skip this step, we are left with huge dry cleaning bills so they could save a few dollars and make us think we are. Also, on clothing that is washable, you will often find the the seams are a just little gathered coming out of the dryer, or that the garment is not quite on grain like it was (that why our garments tend to look old quickly). The best way to handle these is to gently tug on each end of the seams (particularly in knit fabrics) and straighten it out while warm. The lay the garment out flat or neatly fold it without wrinkles and let it set until it cools completely. If the garment is twisted off grain, gently tug on the two shorter corners to pull it back into alignment and then proceed as above. Your clothing won’t have the advantage of custom-made (and preshrunk) does, but it will look better and last longer.

So, my interpretation here is that, yes, to a degree, if the fabric is pre-washed, it may affect the final outcome's washability... so it's not only the fabric type that determines the washing method on store-bought clothing.
Another interesting source is on The Laundress where they go through various fabric types and garment styles to hash out what can and can't be washed at home. The ones that relate to fabrics specifically include

Don't wash viscose (it shrinks, often unevenly)
Don't wash polyamide blends (they grow)
Don't wash &quot;structured&quot; items - ties, blazers with shoulder pads
Don't wash suede &amp; leather - if marked &quot;not washable&quot; or &quot;Dryclean only&quot;
Don't wash fur with skin

There's actually not too much on this list, amusingly... so it seems that, provided you do some research on how to wash anything else properly (that does not mean you can toss everything in your washer, though), you should be able to avoid dry clean only clothes to a large extent.