You don't really say what your trying to accomplish, so I can't say if this is recommended advice, but 'yes'. Yes, it's possible to just use a text field as the basis of a relationship, if you aren't afraid of doing custom queries.
We've done this in a number of situations. Generally, speaking using the relationship field type or Playa works for us, but every once in awhile, a search:in or custom query performs better.
One example, for use, we needed to be able to display all multilingual versions of a publication. The client, produces publications, not all publications are translated, and not all languages are completed when translation occurs, plus occasionally publications are translated into random languages.
So a publication could have an English, Chinese, Spanish, Vietnamese, or Korean version (the five standards), but occasionally may have a Russian, Hmong, or other language translation as well.
We made the relationship field, a text field that already exist, PubID. This field exist to correlate the web version of the publication to the print version, but any version of the publication got the id.
We have six language channels— english, spanish, chinese, korean, vietnamese, and other. We're using a custom query, since the system was set-up pre-search:in days, but basically the query just looks for the current PubID across all six channels, returns the channel name. A subquery is then executed if the channel name is 'other', to pull the results from the specified language field.
So whatever version of the publication a user is viewing, a listing of all the other languages it is available in is provided, and the client doesn't have to keep an updated language list, they just publish the new version, when and if it's ever translated.
So that's a rather long statement to say, 'yes' if the situation warrants it, it is doable, but make sure doing it, makes life easier for the project/client.