If I need to fit a quotation syntactically into a sentence, can I use empty brackets to indicate that I have removed letters from a verb?

No. In MLA style, brackets are generally only used to add material or show visible alterations, not to indicate omissions.1 So when attempting to fit a quotation syntactically into a sentence, you must find a different solution.

Let’s say, for example, you want to quote the opening sentence of David Lodge’s novel Changing Places:

High, high above the North Pole, on the first day of 1969, two professors of English Literature approached each other at a combined velocity of 1200 miles per hour. (7) 

If the syntax of your sentence demands that you place the quotation in the present tense, do not replace the -ed at the end of “approached” with empty brackets: 

At the beginning of David Lodge’s novel Changing Places, “two professors of English Literature approach[] each other at a combined velocity of 1200 miles per hour” (7).

Although it would not be wrong to place the entire word in brackets to show that you have altered it, doing so risks ambiguity. The reader is likely to think “approach” is your word, not Lodge’s:

At the beginning of David Lodge’s novel Changing Places, “two professors of English Literature [approach] each other at a combined velocity of 1200 miles per hour” (7).

In this case, the best solution is to paraphrase the troublesome word:

As David Lodge’s novel Changing Places opens, “two professors of English Literature” are coming toward “each other at a combined velocity of 1200 miles per hour” (7).

Note 

1. Legal style, explained in The Bluebook, permits empty brackets to indicate “the omission of letters from a common root word”—for example, “judgment[]” (77).

Works Cited

The Bluebook: A Uniform System of Citation. 19th ed., Harvard Law Review Association, 2010.

Lodge, David. Changing Places: A Tale of  Two Campuses. 2nd ed., Penguin Books, 1979.