it is so hard when a child can not speak! I am glad to hear that her receptive language is intact, and that she is able to participate successfully in the mainstream setting. I wonder what is the official diagnosis for why she can not speak. Some children have motor control issues such as dyspraxia or hypotonia, sometimes children have had strokes that impacted the areas of the brain that allow us to speak, so I am wondering exactly what is preventing your daughter from speaking (for more on language delays, visit www.asha.org). Since she has not been making progress, I would ask your pediatrician for a referral to a pediatric neurologist to learn more. I would also ask for a referral to a pediatric neuropsychologist in the event that she has never had a neuropsychological assessment. She is old enough now for these professionals to collect informative data that will help you determine what her needs are.
You did not mention your daughter being autistic, but I do wish to address the issue of DMG and other 'alternative' treatments for children with disabilities. There are many recommendations out there for treatments for children with disabilities, and in particular recommended for children with autistic spectrum disorders. Even if a person has a Ph.D. or MD, stay skeptical about what you see in books (as opposed to peer reviewed research journals such as The New England Journal of Medicine). I would not recommend you give your child any drug/supplement/treatment that has not been rigorously tested and found to be safe and effective. Remember, that if something is strong enough to help you, it is strong enough to hurt you (even familiar drugs like Tylenol and aspirin can cause nasty problems).
While some of these treatments that have been recommended are not necessarily harmful, when subjected to scientific inquiry many have been found ineffective (e.g. casein free diet, Secretin). Please keep in mind that a nutritionist is not trained at the level of your daughter's pediatrician, and you should not make medical decisions without consulting a medical doctor.
The American Academy of Pediatrics website (www.aap.org) as well as the Association for Science in the Treatment of Autism wil have trustworthy, up to date information about which treatments are effective and which are not (http://www.asatonline.org/resources/makingsense.htm)
Okay, this is way out there, but you did say any ideas, so here goes. There is a product called Dimethylglycine, DMG, that has been used with good results in children who are speech delayed. It is technically classified as a food and is very safe. Speech is the most notable positive change in the children who are helped by DMG, but behavioral improvements have also been reported.
I have come across this information in a few books, including Biological Treatments for Autism and PDD by William Shaw, Ph. D. I think the authors of this book recommend trying DMG for at least two or three weeks, although many children began speaking within the first week of taking it.
If you're interested in trying it, you might ask a nutritionist about it first, or do some research on the net and see if you think your daughter might benefit from it. Even though it's safe, I wouldn't know what amount she should take. Probably best to handle it through a registered nutritionist who's heard of using DMG this way.
I hope you find something that helps.