As long as you don't claim each other as spouses in a legal context (taxes, insurance forms, etc) it's hard to prove any crime. You can commit yourself to whomever you like as long as you don't try to use it legally.
I did a handfasting at my (legal) wedding. I didn't find it to be flaky or transitory or anything. Historically they were used as temporary in cases where bearing an heir was a necessity. The words we chose to spoke showed our commitment to one another, but also acknowledged that the marriage/relationship will only last as long as we are both in love. I don't regret including that at all.
You can do everything like a traditional wedding except for signing a marriage certificate. Do whatever you would do if you were going about it for a "normal" marriage. Enjoy your day. Do everything you dream of doing. Just don't include the government or any paperwork that could be considered proof of marriage and you're good.
The only other way it can be construed as illegal is under the bigamy or adultery laws. These vary by country and by state in the US. Most states don't do anything about them unless they get a lot of complaints about a particular family. As long as you are low profile (i.e. not on national television and such), the likelihood of their being legal repercussions for breaking that law is slim.