Methadone is extremely effective orally. Given how constipating methadone is, you're likely to lose quite a bit if you try rectal as it might well get absorbed by faeces, unless you give yourself a good thorough enema beforehand.
I've taken a double dose of my methadone myself in the past. All it did was make me tired yet kinda restless, itchy and warm and I sweated like a bastard, seriously, sweat beaded on my forehead and face just from standing about and walking. Got insomnia too which was frustrating given the fatigue I was feeling. In short, it wasn't worth it and was a waste of the methadone. The next 2 days after a double dose, my usual dose didn't hold me as long. Nothing major but I definitely felt a bit off.
Really you shouldn't consider using your takehomes to get high. If you're still having cravings, tell your drugworker and get your dose upped. The "best" way to use it to get high is to stash your dose and use gear that day instead. Not having methadone in your system that day will make the rush and high of the heroin better and now you've extra methadone for a rainy day. It's poor HR advice, but if you're not totally ready to stop getting high, that's OK, it can take a long time to get there. Using heroin 2 times a week and methadone 5 days is better than using heroin everyday of the week, but associating methadone treatment with getting high can really defeat the purpose of the treatment