您可以使用ControlValueAccessor接口来自定义Angular Material表单字段控件，并在其中访问NgControl。以下是一个示例解决方案：

首先，创建一个自定义表单字段控件组件，实现ControlValueAccessor接口：

import { Component, forwardRef, Input, OnInit, Provider } from '@angular/core';
import { ControlValueAccessor, NG_VALUE_ACCESSOR, NgControl } from '@angular/forms';

export const CUSTOM_INPUT_CONTROL_VALUE_ACCESSOR: Provider = {
  provide: NG_VALUE_ACCESSOR,
  useExisting: forwardRef(() => CustomFormFieldComponent),
  multi: true
};

@Component({
  selector: 'app-custom-form-field',
  template: `
    
      
    
  `,
  providers: [CUSTOM_INPUT_CONTROL_VALUE_ACCESSOR]
})
export class CustomFormFieldComponent implements ControlValueAccessor, OnInit {
  @Input() placeholder: string;
  @Input() disabled: boolean;

  value: any;
  onChange: any = () => {};
  onTouched: any = () => {};

  constructor(private ngControl: NgControl) {}

  ngOnInit() {
    // 注册控件访问器
    this.ngControl.valueAccessor = this;
  }

  writeValue(value: any) {
    this.value = value;
    this.onChange(value);
    this.onTouched();
  }

  registerOnChange(fn: any) {
    this.onChange = fn;
  }

  registerOnTouched(fn: any) {
    this.onTouched = fn;
  }

  setDisabledState(isDisabled: boolean) {
    this.disabled = isDisabled;
  }
}

然后，在使用该自定义表单字段控件的父组件中，您可以通过ViewChild装饰器访问NgControl：

import { Component, OnInit, ViewChild } from '@angular/core';
import { NgControl } from '@angular/forms';
import { CustomFormFieldComponent } from './custom-form-field.component';

@Component({
  selector: 'app-parent-component',
  template: `
    
    Log NgControl
  `
})
export class ParentComponent implements OnInit {
  @ViewChild('customFormField', { static: true }) customFormField: CustomFormFieldComponent;

  constructor() {}

  ngOnInit() {}

  logNgControl() {
    console.log(this.customFormField.ngControl);
  }
}

在上面的示例中，我们通过ViewChild装饰器将CustomFormFieldComponent实例赋值给customFormField变量，并通过customFormField.ngControl访问NgControl实例。

这样，您就可以在自定义表单字段控件中访问NgControl实例，并执行需要的操作。