I would suggest you add lactic acid after your mash (direct into boil kettle?) With a properly designed sour beer you will mash in the optimum ph range. You will have some undesirable consequences from mashing at far too low ph.
Have you thought about souring by adding lactic bacteria to your mash tun for ~36 hrs (Berliner Weiss procedure ). You then boil to kill the bacteria but retain the sourness.
Acidulated malt is just malt that's been treated with lactic acid. So you'd get the same effect as adding lactic acid directly to your beer/wort. I've never done this, but the general consensus is that it's a bad idea. Generally lactic acid alone is very one dimensional and even chemically tasting.
Like I said, never done it... But it's been asked plenty of times on the 'Milk the Funk' Facebook group (do a search).
Kettle souring though, is something I strongly endorse
It depends what you are after. If you want the refreshing tartness of a Berliner Weiss you can get something similar, but you will never get the funkiness of a beer fermented with bugs. A plastic fermenter is cheap. Set one aside just for use with sour beers then you can be sure to have no cross contamination.
Just take the plunge and use a lacto culture, you won't regret it. As dannymars has said, it will be very one dimensional, chew on some acid malt because thats what it will taste like. You'll be freaking out the first time you use any kind of sour bugs but you get over it!
Re- chew on some acidulated malt. That's what it will taste like. Its not nice. I did a red ale with acidulated to 8% (far too much) before I had any experience with it and it spoiled the brew. Acidic tartness.
I only use it to get correct mash pH. Up to 3%.
If I read the specs correctly, you'll end up with a mash pH around 3.5 and about 5-6 g/l of lactic acid contributed by the malt.
The first will create some difficulties as it's below the optimum for enzymatic conversion but in general low pH is less of a problem than high, so if you doubled your mash dwell lengths you'd probably be OK.
It will also reduce the solubility of alpha acids and the rate of iso alpha conversion but you probably don't want much bitterness in a beer this sour anyway.
Assuming your beer would have had about 1 g/l of organic acids anyway you'd end up around 6-7, which appears to be in the usual range for sours.
BTW I thought German acidulated malt was produced by a lactic fermentation of wort which is then sprayed onto the malt before kilning. My understanding is that this gets around the reinheitsgebot strictures on additions to beer.
It might sound like a crazy enough to work but if it did - like rapid souring - wouldn't everyone do it? I mean why wait 4 months for some Lactobacillus and Brettanomyces to work their way through your wort when you could brew a 50:50 pils:wheat to 3% then stir in some lactic acid? Because it isn't that simple, the bugs don't just spit out some lactic acid and disappear into the trub.
In a brew there are two main times the pH is critical: mashing and fermenting. Home brewers often forget about the pH of the ferment (or altogether) but with a sour brew, acid is sometimes added after the boil to adjust the pH to suit the bacteria. If it were that easy to make a sour brew then skip the bacteria altogether, add some acid post-ferment and enjoy a non-authentic sour fast and easily. But I confident you won't get the beer you're after.
I've done a brew with 7ml of lactic acid and it was rubbish. I made a few other errors but I definitely know the contribution the acid made. If you want to know similarly then I suggest adding some lactic acid directly to a glass of beer (0.1ml will do) and see what it contributes.
That said happy to be wrong and I would genuinely love to see you brew with a massive quantity of acid malt. Make sure you report the progress.
I did a Berliner Weiss a while ago (no pH meter at the time though). Mash with 50/50 pilsner Malt / Wheat then dropped the temp to 40c. Threw in a handful of fresh grain and held at 40c for 48hr. After this boil to kill the bugs then ferment. Turned out great, tartness similar to a white wine.
Having said that I agree with huez. Just set aside a fermenter for bugs. It will be a lot more complex than you could ever get with acidulated malt or lactic acid.
I did mine in an esky. Threw a heat belt in connected to my ferment fridge thermostat. You could do the same with your BM, just put something around it like a blanket for some insulation. It is important to have an oxygen barrier as you want anaerobic activity. I did this with cling wrap over grain but you can use a co2 blanket if you run a draught system.