Is it frowned upon in Buddhism to undertake homelessness without ordination? Let us presume that part of this homelessness also includes a focus on meditation, with mind and body able to work for oneself.
Homelessness is simply freedom from home-boundedness.
Homelessness is too negative a term. Those who wander free of homes do have their concerns, such as deciding in which beautiful place to sleep tonight, but this is minor compared to the distractions of 30-year mortgages, ever-growing utility bills, the complexities of adequate property insurance, too many neighbors, the hope of an adequate retirement many, many years from now...
One path is not better than the other, and haughty frowns of judgement may be thrown in either direction. One thing is certain: Do not choose your path based on who may or may not frown at you!
A time of homelessness (as the home-bound like to call it) has been an essential period in the lives of many great seekers, the Buddha, Lau Tzu and Jesus being only a few obvious examples. The key is to be homeless with awareness, but this is equally true for the home-bound.
Yes. It's frowned upon generally in Buddhism to practice on your own if suitable teachers are available. Dogen (the Zen founder) stressed the importance of finding and studying with a teacher, so did Ajahn Chah (a teacher in the Theravadan tradition):
However, if one only has knowledge of books and scriptures, sermons and suttas, that is, only knowledge of the map or plans for the journey, even in hundreds of lives one will never know purity, radiance and peacefulness of mind. Instead one will just waste time and never get to the real benefits of practice. Teachers are those who only point out the direction of the path. After listening to the teachers, whether or not we walk the path by practicing ourselves, and thereby reap the fruits of practice, is strictly up to each one of us.
If you just want to do intensive meditation I think you would quickly find homelessness (especially homelessness without the support of a sangha) was more of a hindrance than a help.
The lesser knowing and understandig the teachings of the Buddha and the more people are into "Buddhist" and "Buddhism" they lesser they accept homeless ascetics, even if having taken refuge into the three gems. The therm Anagarika seems to be a special Sinhalese rendering and is merely unknow in other Buddist countries. A person still taking money, using money, can not really be called a homeless, recluse or Samana.
For the Sangha, other recluses should be no problem, and either they speak Dhamma or not, would lead them in regard of respect. There are serial rules dealing how Bikkhus have to handle with not ordained beggars.
One thing one should be aware, one does neither have a/the community of monks as support or refuge nor sociaty, so one needs a lot of confiction and resolution to walk such a way.
One should be also aware, that if thinking in ways "I don't like to work, I have no support, what if I wear the Robs of the Bhikkhus and mingle my self under the Bhikkhus, since they are well supported". In such case, out of such an intention, Bhikkhus would not be allowed to ordain someone having cheated being a full ordained Bhikkhu before while not.
Very practical it's of course good to have a community somehow behind, on the other hand such also has its limits of benefit an disadvantages.
Its not easy today and was also not easy in times of the Buddha, incl. the Buddha him self, living as real homelessness without a communal support and big reputation and adverticing behind. So it's really not a "let's see what comes along" life for certain periode of time.
How ever, there are less wise and knowledgeable people, whether members of the Sangha nor housholder, becoming aware of an recluse, who do not highly respect really homeless beggars on a/the path for liberation.