Methaqualone and ethaqualone don't have protonatable nitrogens and can't form salts.
Dont inject quinazolones. And don't inject alcoholic solutions either.
Methaqualone and etaqualone can form salts. Parest, Somnafac and Optimil had 400mg of methaqualone hydrochloride. Tuazole and Biphetamine-T were the resinate for extended release. The hydrochloride is actually weaker and less lipid soluble than the freebase. This is probably why brands with the freebase(Mandrax, Sopor, and Quaalude) were more popular even though the didn't have as high a dose.
Even the hydrochloride of methaqualone is only soluble in a hot acidic water or alcohol, not anything you'd want to inject. Don't know about other salts. However with methaqualone it has been used IV
http://bja.oxfordjournals.org/content/44/1/83.short . I'm not sure what they used to dissolve it. I think I read somewhere that they used 300mg, can't find the full article now. However, they were anesthetized quickly, with muscle spasms at first, then woke up after 10-15minutes. I'd agree that etaqualone shouldn't be inject without more research. General anesthesia isn't really something to do at home.
Methaqualone itself was only half as toxic as the least toxic barbiturate phenobarbital and was less likely to cause respiratory/cardiac depression by itself. However it was more likely to cause seizures and bleeding. With methaqualone the biggest killers were car accidents, non-vehicular accidents, homicides, and suicide, not so much accidental OD. So don't drive, do something stupid or get into fights on etaqualone.
Supposedly mecloqualone was much less toxic then methaqualone. I can't find shit on ethaqualone even though it was used medically. It might not be as "safe" relative to methaqualone or mecloqualone, not that they're safe in the first place. It did almost kill a member of this site.