Was there ever a study done on the percentage of Reform Jews who would be considered Jewish according to Halacha?
No, and it would be nearly impossible to determine. Every modern survey and census of Jews in America has been performed with the widest possible definition of Jew, in order to obtain the fullest and least-controversial numbers. This usually translates to counting someone as a Jew if they identify themselves as Jewish. (Source)
For example, the most recent comprehensive census done, the National Jewish Population Survey 2000-01, primarily counted someone as a Reform Jew if they considered themselves to be one. The more stringent definition was simply those who are members or are affiliated with to a Reform synagogue. (Source) Neither definition gets anywhere near "Halakhic."
Even if one were to attempt to find out, they'd run into the following problems:
Reform Judaism accepted Patrilineal Descent in 1983. Any survey or census performed since will have been done according to their standards, which means that the latest generations of Reform Jews will only needed to have just one Jewish parent, and the data will not distinguish which parent it is.
Also, Reform Judaism counts the children of converts as Jews. However, Reform conversions are not performed to traditional halakhic standards, rending their children non-Jewish according to Halakha.
Finally, even if the mother was halakhically Jewish, there are many problems with potential mamzeirut, as Reform Judaism does not conduct halakhically valid marriages and divorces.
While, no exact number can be known, estimates are possible.
In a survey done with Reconstructionist, Conservative, and Reform rabbis, no Conservative rabbis would perform an intermarriage, 36% of Reform rabbis would, and 62% of Reconstructionist rabbis would.
In a population survey, 33% of American Jewish Families were "interfaith families", while another study says that currently, just over 50% of marriages are "interfaith marriages"
According to this article, It's hard for me to tell if 33% of intermarried Jewish families had a Jewish mother, or if 29% of Jewish woman, and 33% of Jewish men were intermarrying. The quote is..
Based on this information, About 18% of Reform Jews today would not be halachically Jewish. And 33% of Reform children born now, would not be halachically Jewish.
While not specific to Reform Jews, the National Jewish Population Survey from 2000-01 (which was just enough time after Patrilineal Descent for college kids to have been born) the found that only 48% of Jews in college had two Jewish parents. Which would mean that 26% of Jews in college have only a Jewish father, and thus aren't halachically Jewish (and this is even assuming that all self-identified Jewish parents are halachically Jewish). Now if that was the case 13 years ago and intermarriage rates have increased or at least stayed the same, and the problem compounds by being left unchecked, then the current and future generations of Jews are going to have even higher percentages of non-Jews in their midst.