Didn't the government renege on gold-clauses however? That's not a stellar example. Anything more recent than the 1870's or 1930's? I understand the part about contractual obligations, which is why I diverted the discussion to tax liabilities. Presumably the coin of the realm ought to be in some manner acceptable to those levying the taxes. Do you disagree?
As for taxes, the general rule (you almost always have to say "as a general rule," because the law for better or worst generally contains exceptions, exceptions to the exceptions, and exceptions to the exceptions to the exceptions, ad nauseum) as I understand it is that the federal government has to accept all legal tender in payment of TAXES.
Despite the general rule, I'll bet that there is some point at which an outlandish form of payment using legal tender would be enough to make a judge penalize the payer.
As to past taxes, there are a couple of lines of cases that distinguish between tax payments to the government and the payment of license and similar "non-debt" fees, stating that the federal government doesn't have to accept all forms of legal tender in payment of license and non-tax fees. Do they all appear to be consistent? Not to me.
I think you hit it on the head when you said "in some manner acceptable" to those levying the taxes. A governmental body would probably be within its rights to require you to roll your coins, or to make you wait while the coins are counted if you insist in paying with them. Some states, like California, have regulations that say a governmental agency does not have to accept coins. Constitutional? Don't know; I can't find anywhere where that a similar law has ever been tested. My guess is that if the law doesn't pose an unreasonable burden on the payer, it would probably be held constitutional.
The problem with large payments in coin is that they are almost always intended to cause inconvenience to the recipient. Judges see through that. If the inconvenience is too great, there's likely to be some penalty assessed against the person creating the inconvenience.
In a vacuum, many prople don't see a problem with somebody paying in pennies. Sometimes the government NEEDS somebody to stick it to them. Some protests are valid. Some inconveniences are minor. We can all live with those. However, if somebody in front of me at the DMV tried to pay a $478.32 ticket in pennies (my dentist, for example, who honestly believes that dentists and physicians should never be given speeding tickets because of their special status in society) and caused me to wait in line an extra three hours for them to be counted, then I would view her protest in a dim light.
One of the difficult points of the U.S. "common law" system is that lines are constantly being drawn and redrawn, and appellate judges are SUPPOSED to identify why a line is drawn and where.
For example, should you be permitted to pay in currency covered in mayonnaise? What about wet currency that has to be laid out, unstacked, to dry without molding? Shredded currency? Coins frozen in a large block of ice or chili? Should a tax office be required to accept a $10,000 bill (Salmon P. Chase on its face) in payment of a $1.16 tax bill? How about if you take each bill and cut 45% of it off? A bill with over 50% still present is worth its face value. How much should the payer be able to inconvenience the recipient of payment? That's what the laws and restrictions are about.