Hawaii's Sen. Daniel Inouye is tied up these days as co-chairman of the Iran-contra hearings, but several thousand pitiful and helpless human beings would like to send him a message: get on with those hearings, senator. You have other important work to do.

Inouye is principal sponsor of a bill, S. 143, that he hopes will become the ''Compassionate Pain Relief Act'' of 1987. The bill would establish ''a temporary program under which parenteral (injectable) diacetylmorphine will be made available through qualified pharmacies for the relief of intractable pain due to cancer.''

Diacetylmorphine is heroin. For persons dying of inoperable cancer, many physicians believe it is the one drug that would best relieve the agony their patients suffer.

The idea of making heroin legally available to such victims is an idea that may be hard to accept, but it is scarcely a novel idea. Physicians in Great Britain have been prescribing heroin for such patients for several years. It works.

Inouye's bill sets forth certain findings that lay the groundwork for his proposal. Cancer afflicts one out of four Americans; it is the second leading cause of death. ''In the progression of terminal cancer, a significant number of patients will experience levels of intense and intractable pain which cannot be effectively treated by presently available medication.'' The effect of such pain often leads to ''a severe deterioration in the quality of life of the patient and heartbreak for the patient's family.''

At present, any use of heroin -- even therapeutic use -- is prohibited by law, although extensive clinical research has demonstrated the remarkable painkilling properties of the drug. The reasons that are given for opposing Inouye's bill lack both substance and merit.

Opponents contend, for example, that if heroin is legally stocked in a hospital's pharmacy, attempts will be made to steal it. This is a possibility, of course, but it can be dealt with. A related objection is that a pharmacy's supply of heroin might be diverted in some fashion to someone other than the patient for whom it is prescribed. The National Committee on the Treatment of Intractable Pain makes the point that if all the heroin required under Inouye's bill were stolen or diverted -- all of it -- it would constitute only 2 percent to 4 percent of the heroin illegally on the street and available to addicts.

Another objection -- and this seems especially callous -- is that a cancer patient might unexpectedly survive and turn out to be addicted. The bill is intended for the relief only of those patients suffering cancer ''with a high and predictable mortality.'' There would be time to worry about addiction if the terminally ill patient surprised his doctors.

When the senator's bill was under discussion in the 99th Congress, some physicians testified that heroin is no better than other drugs for the relief of agonizing pain. The response to this objection seems to be yes and no. When given by mouth, in single doses, heroin and morphine may work equally well, but when given by injection, heroin may be more effective in certain patients. As for Dilaudid, it carries side effects that terminally ill patients positively do not need.

A final group of objections, gleaned from last year's debate, stemmed from the hopeful notion that eventually a new non-addictive drug, equally as effective as heroin, will be developed. Perhaps so, but for a patient suffering the kind of pain with which the bill is concerned, ''eventually'' is a poor substitute for ''now.''

The principal, unstated objection to the Inouye bill is that members of Congress do not want to be politically identified with the legalization of heroin under any circumstances. This imputes to members both a want of compassion and an excess of cowardice. Surely voters are capable of understanding the vast difference between promoting heroin and prescribing it.

Inouye's bill contains abundant provisions against abuse. The drug would be available only to a special class of patients; it could be dispensed only by written prescriptions from licensed physicians. The program would expire after five years if it turned out to be unworkable.

Isn't this worth a try? Three times in my life I have seen loved ones die of cancer, and two of them were in such pain they could neither weep nor scream. Injections of heroin might have let them go in relative peace. It's not a great deal to ask.