^^^No, codeine probably wont do shit, although if your only on 20-30mg it may do something.
The problem with taking other opiates/opioids with methadone, is the competition for receptor space. Most things cant compete with methadone, but some can, and if you are a chronic MMT user, you make experience withdrawal.
And its somewhat of a myth that methadone "blocks" other opioids. Its simply a matter of methadone having an EXTREMELY high affinity to the mu receptors, and it takes quite abit to overcome it. But it really is simply a matter of tolerance, and methadone creates a very strong tolerance. And there is no "set" dose where other opioids stop working when taking methadone. Again its just a matter of tolerance, and if you take enough of a strong opioid, you can bypass any amount. For example, when i was on 180mg, i would shoot .8-1.2g of tar heroin per shot, and i would get VERY high. I am on 150mg now, and i shoot .5g of better quality heroin 2-3 times a week, and i get high, enough to nod usually. Same with shooting two 10mg duragesics, which would get me high on 180mg of methadone. Levorphanol also seems to work very well in permeating methadones strong-hold on the receptors. Also one of the best way of beating methadone's tolerance, is by taking more methadone. Doubling your dose can get you very high, as methadone works very well ontop of methadone...
So really, its simply a matter of dosing high, as there is no dose of methadone that you can not get high on if you just use enough heroin or whatever strong opioid (levorphanol, heroin, fentanyl analogues). Also, in attempting to defeat the high dose methadone tolerance, its best to try to get high when you are the furthest away from when you took the methadone, so the methadone is well past its peak and its easier to surpass. This is why i shoot heroin at night, as if i were to shoot the same amount of heroin 5 hours after taking my methadone i would not experience the same high. But there is definately no dose that methadone cannot be overpowered, you just have to becarefull of the possibly fatal respiratory depression. Tolerance to euphoria is DEFINATELY different than tolerance to respiratory depression, and tolerance to euphoria grows far faster than the tolerance to the respiratory depression. They definately dont grow at the same rate, so keep this in mind.....
Also about dosing, I have been up to 240mg/day, taken orally in one morning dose. Supposedly the legal limit in my state is 180mg, but there is a DEA form to allow doses above this, which i had in my case. So doses up to 240/250mg are not impossible, its just not very common. Doses up to 300-400mg have been heard of, but again, not common. But i wasnt at 240mg for much longer than 60 days; it was definately getting me high, i have to admit that....