Whyever not? Sure, you can't use other gadgets but why would you think that you couldn't use side reins - but only when lunging and never with a rider on!
i know that you definitely can but i understand why some people might think you can't- with some horses they do really help to get them to soften and relax before having to do a DR test (in the same way that another 'gadget' might help in ridden work)...i am so glad you can as my horse goes really nicely after 5/10 minutes in side reins...