Haven't tried it, but it was tried for MG many years ago and some folks do still try it. It has some major differences from Mestinon.
"Galantamine crosses the blood brain barrier and concentrates in the brain" says the reference I linked below. Mestinon does not.
I think I would be worried about what would happen if this accumulated in my brain -- MG is not a brain disease, but rather a neuromuscular junction disease.
I don't think I would want to experiment with brain altering chemicals unless my brain were in need of alteration ;-)
It is used for treatment of Azheimers because it goes into the brain and increases the level of Acetylcholine there.

https://books.google.com/books?id=TU37CAAAQBAJ&pg=PA25&lpg=PA25&dq=galantamine+myasthenia+gravis+research&source=bl&ots=D1TTY7xgZM&sig=PhWkdH0qC3PT7xNCEl6jAtRDnv4&hl=en&sa=X&ved=0ahUKEwi844j1-s7TAhUq64MKHRiRAfY4ChDoAQglMAE#v=onepage&q=galantamine%20myasthenia%20gravis%20research&f=false

Here are some of the side effects of high levels of acetylcholine in the brain. An impressive list:
Symptoms of High Acetylcholine

Subdued or depressed mood
Anhedonia (inability to experience pleasure)
Difficulty concentrating
Difficulty with higher-order or complex thought processes
Mental fatigue
Mental confusion
Memory problems
Decreased motivation
Feeling overly sleepy or tired (particularly in the evening), despite adequate sleep and rest
Difficulty understanding or performing tasks
Pessimistic, negative ideation or rumination
Feelings of helplessnes and hopelessness
Fretfulness
Irritability or anger
Emotional heightening and lability
Sadness, tearfulness
Blurred vision
Headache
Dry mouth
Altered sense of smell; heighted sensitivity to odors; olfactory delusions
Stomach pain or discomfort
Intestinal gas or bloating
Diarrhea or constipation
Nausea, dizziness, and vomiting
Muscle pain or discomfort
Joint pain, discomfort, or swelling
Tooth or jaw pain or discomfort
Tingling or numbness in arms or legs
Muscle weakness
Increased urinary frequency or problems with bladder control
Flu or cold-like symptoms
Weakening of immune system and increased susceptibility to illness
Cough
Nasal discharge
Chills or sensation of cold
Cold feet, hands
Sleep disturbance
Introversion
Anxiety
More and vivid dreaming, and higher incidence of nightmares
Decreased levels in the brain of the neurotransmitters, serotonin, norepinephrine, and dopamine
Interference with the release of the above brain transmitters, because of higher inhibition

This is incredibly helpful! Thank you. Wow. That is a very, very long list of side effects. Thank you for the link, too. -Julie

I am undiagnosed, but have all the symptoms and since I wasn't getting anywhere in the system I decided to try huperzine since you can buy it over the counter. I became mostly stable when I started taking huperzine. The improvements were dramatic. It works via the same mechanism as mestinon, has less side effects (haven't experienced any), lasts longer, no rebound effect. And it's inexpensive too!

According to the wikipedia entry on huperzine, it too crosses the blood brain barrier. The problem with supplements, are they are not regulated, tested, researched very well, and so one never really knows if they are getting what the label says. Also no studies of the long term effects of taking it have ever been done -- the research studies are about 3 months long and related to Alzheimer's or brain functioning.